diff --git a/apps/filmcraft/src/audio.rs b/apps/filmcraft/src/audio.rs
index 4e54897c..914f10f8 100644
--- a/apps/filmcraft/src/audio.rs
+++ b/apps/filmcraft/src/audio.rs
@@ -12,6 +12,7 @@ pub struct CpalOut {
     stream: Option<cpal::Stream>,
     played: Arc<AtomicU64>,
     rate: u32,
+    channels: u16,
     hw: AudioHardwarePrefs,
     /// Sequence sample rate for "Attempt to force hardware to document sample rate".
     document_rate: Option<u32>,
@@ -44,7 +45,14 @@ impl CpalOut {
         let host = cpal::default_host();
         let dev = host.default_output_device()?;
         let cfg = dev.default_output_config().ok()?;
-        Some(Self { stream: None, played: Arc::new(AtomicU64::new(0)), rate: cfg.sample_rate().0, hw: AudioHardwarePrefs::default(), document_rate: None })
+        Some(Self {
+            stream: None,
+            played: Arc::new(AtomicU64::new(0)),
+            rate: cfg.sample_rate().0,
+            channels: cfg.channels(),
+            hw: AudioHardwarePrefs::default(),
+            document_rate: None,
+        })
     }
 
     /// The stream configuration the settings ask for, falling back to the device default.
@@ -77,6 +85,7 @@ impl AudioOut for CpalOut {
             config.buffer_size = cpal::BufferSize::Fixed(self.hw.buffer_size);
         }
         self.rate = cfg.sample_rate().0;
+        self.channels = cfg.channels();
         self.played.store(0, Ordering::SeqCst);
         let played = self.played.clone();
         let err = |e| eprintln!("filmcraft: audio stream error: {e}");
@@ -103,6 +112,9 @@ impl AudioOut for CpalOut {
     fn sample_rate(&self) -> u32 {
         self.rate
     }
+    fn channels(&self) -> usize {
+        self.channels as usize
+    }
     fn played_frames(&self) -> Option<u64> {
         self.stream.as_ref().map(|_| self.played.load(Ordering::SeqCst))
     }
@@ -122,6 +134,7 @@ impl AudioOut for CpalOut {
             && let Ok(cfg) = self.config(&dev)
         {
             self.rate = cfg.sample_rate().0;
+            self.channels = cfg.channels();
         }
     }
 }
diff --git a/crates/aac/src/decoder.rs b/crates/aac/src/decoder.rs
index b112eb37..14dbbf4e 100644
--- a/crates/aac/src/decoder.rs
+++ b/crates/aac/src/decoder.rs
@@ -19,6 +19,10 @@ const ID_PCE: u32 = 5;
 const ID_FIL: u32 = 6;
 const ID_END: u32 = 7;
 
+/// `extension_type` of a fill element carrying SBR data (ISO/IEC 14496-3 §4.5.2.8.2, Table 4.121).
+const EXT_SBR_DATA: u32 = 13;
+const EXT_SBR_DATA_CRC: u32 = 14;
+
 /// One decoded `individual_channel_stream` before stereo processing and synthesis.
 struct Ics {
     info: IcsInfo,
@@ -45,6 +49,8 @@ pub struct Decoder {
     offsets: Vec<usize>,
     channels: Vec<ChannelState>,
     noise_state: u32,
+    /// A fill element carried SBR data (implicitly signalled HE-AAC).
+    sbr_seen: bool,
 }
 
 impl Decoder {
@@ -67,7 +73,7 @@ impl Decoder {
         if layout.is_empty() && asc.channel_config != 0 {
             return Err(Error::InvalidConfig("unsupported channel configuration"));
         }
-        let mut d = Decoder { asc, layout: Vec::new(), offsets: Vec::new(), channels: Vec::new(), noise_state: 0x2545_F491 };
+        let mut d = Decoder { asc, layout: Vec::new(), offsets: Vec::new(), channels: Vec::new(), noise_state: 0x2545_F491, sbr_seen: false };
         d.set_layout(layout);
         Ok(d)
     }
@@ -90,6 +96,12 @@ impl Decoder {
     pub fn sample_rate(&self) -> u32 {
         self.asc.sample_rate
     }
+    /// Whether the stream is HE-AAC: SBR signalled in the `AudioSpecificConfig` (explicit) or SBR
+    /// data seen in a fill element of a decoded access unit (implicit). Only the AAC-LC core is
+    /// decoded, at [`Self::sample_rate`]; the SBR high band is not reconstructed.
+    pub fn sbr(&self) -> bool {
+        self.asc.sbr || self.sbr_seen
+    }
     pub fn channels(&self) -> usize {
         self.channels.len()
     }
@@ -174,7 +186,11 @@ impl Decoder {
                         count += br.read_bits(8)? as usize;
                         count -= 1;
                     }
-                    br.skip(count * 8)?;
+                    if count > 0 {
+                        let ext = br.read_bits(4)?;
+                        self.sbr_seen |= matches!(ext, EXT_SBR_DATA | EXT_SBR_DATA_CRC);
+                        br.skip(count * 8 - 4)?;
+                    }
                 }
                 _ => break, // ID_END
             }
@@ -542,3 +558,27 @@ fn synth(state: &mut ChannelState, info: &IcsInfo, spec: &[f32]) -> Vec<f32> {
     state.prev_shape = cur;
     out
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Implicitly signalled HE-AAC (an AAC-LC config) is recognised from the SBR fill element.
+    #[test]
+    fn sbr_fill_element_marks_implicit_he_aac() {
+        // AAC-LC, 22.05 kHz, stereo
+        let mut dec = Decoder::new(&[0x13, 0x90]).unwrap();
+        assert!(!dec.sbr());
+        // ID_FIL, count 1, extension_type EXT_SBR_DATA (+ 4 bits of payload), ID_END
+        let _ = dec.decode(&[0b1100_0011, 0b1010_0001, 0b1100_0000]);
+        assert!(dec.sbr());
+        // other fill payloads (EXT_FILL) don't
+        let mut dec = Decoder::new(&[0x13, 0x90]).unwrap();
+        let _ = dec.decode(&[0b1100_0010, 0b0000_0001, 0b1100_0000]);
+        assert!(!dec.sbr());
+        // explicit signalling: AOT 5, core 22.05 kHz, stereo, extension 44.1 kHz, core AOT 2
+        let dec = Decoder::new(&[0x2B, 0x92, 0x08, 0x00]).unwrap();
+        assert!(dec.sbr());
+        assert_eq!(dec.sample_rate(), 22_050);
+    }
+}
diff --git a/crates/audio-dsp/src/lib.rs b/crates/audio-dsp/src/lib.rs
index 739abeca..977e5a97 100644
--- a/crates/audio-dsp/src/lib.rs
+++ b/crates/audio-dsp/src/lib.rs
@@ -7,6 +7,7 @@
 //!   engine and UI can build controls generically.
 //! * [`biquad`] — RBJ-cookbook biquads (TDF-II) with analytic magnitude response.
 //! * [`sync`] — offset between two recordings of one event (GCC-PHAT, sample-accurate).
+//! * [`resample`] — streaming, block-invariant rate conversion for playback on a device at another rate.
 //! * [`channels`] — channel layouts, ITU-R BS.775 up/downmix, 5.1 mixdown types, the 5.1 panner.
 //!
 //! Conventions: audio is **planar f32** (`&mut [&mut [f32]]`, one slice per channel, all the same
@@ -27,6 +28,7 @@ pub mod fft;
 pub mod loudness;
 pub mod oversample;
 pub mod remix;
+pub mod resample;
 mod smooth;
 pub mod sync;
 
diff --git a/crates/audio-dsp/src/resample.rs b/crates/audio-dsp/src/resample.rs
new file mode 100644
index 00000000..01dfee55
--- /dev/null
+++ b/crates/audio-dsp/src/resample.rs
@@ -0,0 +1,170 @@
+//! Streaming sample-rate conversion for playback, when the output device runs at another rate
+//! than the sequence (a 44.1 kHz device playing a 48 kHz sequence).
+//!
+//! Every output sample's source position comes from its absolute output frame, as an exact
+//! rational (`frame × from / to`), and is linearly interpolated, so the output is identical however
+//! the stream is cut into blocks: no phase jump at block boundaries. Source frames are pulled in
+//! contiguous, non-overlapping spans, so whatever produces them (the mixer, with its effect state)
+//! sees one continuous stream. Output lags the source by one source sample (≈21 µs at 48 kHz),
+//! which is what lets each block interpolate without reading ahead.
+
+/// Converts a planar stream from `from` Hz to `to` Hz, block by block.
+#[derive(Debug, Clone)]
+pub struct StreamResampler {
+    from: u32,
+    to: u32,
+    /// Source frames `[held_start, held_start + held[c].len())` kept from the last pull.
+    held_start: i64,
+    held: Vec<Vec<f32>>,
+    /// The output frame the next block is expected to start at (contiguous playback).
+    next_out: Option<i64>,
+}
+
+impl StreamResampler {
+    /// Rates of zero are treated as 1 Hz rather than dividing by zero.
+    pub fn new(from: u32, to: u32) -> Self {
+        Self { from: from.max(1), to: to.max(1), held_start: 0, held: Vec::new(), next_out: None }
+    }
+
+    pub fn rates(&self) -> (u32, u32) {
+        (self.from, self.to)
+    }
+
+    /// The source frame whose sample lands at (or just before) output frame `out`.
+    fn source_index(&self, out: i64) -> (i64, f32) {
+        let p = out as i128 * self.from as i128;
+        let to = self.to as i128;
+        (p.div_euclid(to) as i64, (p.rem_euclid(to) as f64 / to as f64) as f32)
+    }
+
+    /// Output frames `[out, out + n)`, planar. `pull(start, frames)` returns source frames
+    /// `[start, start + frames)`, planar (missing samples read as silence). A block that does not
+    /// continue the previous one (a seek) starts the stream afresh.
+    pub fn process(&mut self, out: i64, n: usize, mut pull: impl FnMut(i64, usize) -> Vec<Vec<f32>>) -> Vec<Vec<f32>> {
+        if n == 0 {
+            return vec![Vec::new(); self.held.len().max(1)];
+        }
+        let (first, _) = self.source_index(out);
+        let (last, _) = self.source_index(out.saturating_add(n as i64 - 1));
+        // interpolation reads source frames first - 1 ..= last
+        if self.next_out != Some(out) || self.held.is_empty() {
+            self.held_start = first - 1;
+            self.held.clear();
+        }
+        let held_end = self.held_start + self.held.first().map_or(0, Vec::len) as i64;
+        if last + 1 > held_end {
+            let want = (last + 1 - held_end) as usize;
+            let got = pull(held_end, want);
+            if self.held.is_empty() {
+                self.held = vec![Vec::new(); got.len().max(1)];
+            }
+            for (c, h) in self.held.iter_mut().enumerate() {
+                let src = got.get(c).map(Vec::as_slice).unwrap_or_default();
+                h.extend((0..want).map(|i| src.get(i).copied().unwrap_or(0.0)));
+            }
+        }
+        let mut dst = vec![vec![0.0f32; n]; self.held.len()];
+        for (i, frame) in (out..).take(n).enumerate() {
+            let (s, frac) = self.source_index(frame);
+            // held starts at first - 1, so the earlier frame is always held
+            let k = usize::try_from(s - self.held_start).unwrap_or(0);
+            for (d, h) in dst.iter_mut().zip(&self.held) {
+                let a = k.checked_sub(1).and_then(|j| h.get(j)).copied().unwrap_or(0.0);
+                let b = h.get(k).copied().unwrap_or(0.0);
+                if let Some(x) = d.get_mut(i) {
+                    *x = a + (b - a) * frac;
+                }
+            }
+        }
+        // keep what the next block can still read: from its first frame - 1 on
+        let (next_first, _) = self.source_index(out.saturating_add(n as i64));
+        let drop = (next_first - 1 - self.held_start).clamp(0, self.held.first().map_or(0, Vec::len) as i64) as usize;
+        for h in &mut self.held {
+            h.drain(..drop);
+        }
+        self.held_start += drop as i64;
+        self.next_out = Some(out.saturating_add(n as i64));
+        dst
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A sine at `hz`, sampled at `rate`, from absolute frame `start`; logs every pull.
+    fn sine(hz: f64, rate: u32, pulls: &mut Vec<(i64, usize)>) -> impl FnMut(i64, usize) -> Vec<Vec<f32>> + '_ {
+        move |start, frames| {
+            pulls.push((start, frames));
+            let ch: Vec<f32> = (0..frames).map(|i| (2.0 * std::f64::consts::PI * hz * (start + i as i64) as f64 / rate as f64).sin() as f32).collect();
+            vec![ch.clone(), ch]
+        }
+    }
+
+    fn convert(from: u32, to: u32, blocks: &[usize], total: usize) -> (Vec<f32>, Vec<(i64, usize)>) {
+        let mut r = StreamResampler::new(from, to);
+        let mut pulls = Vec::new();
+        let mut out = Vec::new();
+        let mut pos = 0i64;
+        let mut k = 0;
+        while (pos as usize) < total {
+            let n = blocks[k % blocks.len()].min(total - pos as usize);
+            let b = r.process(pos, n, sine(1000.0, from, &mut pulls));
+            assert_eq!(b.len(), 2);
+            assert!(b.iter().all(|c| c.len() == n));
+            out.extend_from_slice(&b[0]);
+            pos += n as i64;
+            k += 1;
+        }
+        (out, pulls)
+    }
+
+    #[test]
+    fn output_is_the_same_however_the_stream_is_cut() {
+        for (from, to) in [(48_000, 44_100), (44_100, 48_000), (48_000, 96_000), (96_000, 48_000)] {
+            let (whole, _) = convert(from, to, &[20_000], 20_000);
+            for blocks in [&[512][..], &[333], &[1, 7, 512, 64]] {
+                let (cut, _) = convert(from, to, blocks, 20_000);
+                assert_eq!(cut, whole, "{from}→{to} in blocks {blocks:?}");
+            }
+        }
+    }
+
+    #[test]
+    fn a_tone_converts_to_the_same_tone() {
+        // 1 kHz at 48 kHz played on a 44.1 kHz device: within linear interpolation's error of the
+        // ideal tone, one source sample late
+        let (from, to) = (48_000u32, 44_100u32);
+        let (out, _) = convert(from, to, &[512], 20_000);
+        for (o, x) in out.iter().enumerate().skip(2) {
+            let t = o as f64 / to as f64 - 1.0 / from as f64;
+            let want = (2.0 * std::f64::consts::PI * 1000.0 * t).sin() as f32;
+            assert!((x - want).abs() < 3e-3, "frame {o}: {x} vs {want}");
+        }
+    }
+
+    #[test]
+    fn the_source_is_pulled_as_one_continuous_stream() {
+        for (from, to) in [(48_000, 44_100), (44_100, 48_000)] {
+            let (_, pulls) = convert(from, to, &[512, 333], 20_000);
+            for w in pulls.windows(2) {
+                assert_eq!(w[0].0 + w[0].1 as i64, w[1].0, "{from}→{to}: {:?} then {:?}", w[0], w[1]);
+            }
+        }
+    }
+
+    #[test]
+    fn a_seek_starts_afresh_and_hostile_input_stays_finite() {
+        let mut r = StreamResampler::new(48_000, 44_100);
+        let mut pulls = Vec::new();
+        let _ = r.process(0, 512, sine(1000.0, 48_000, &mut pulls));
+        let _ = r.process(1_000_000, 512, sine(1000.0, 48_000, &mut pulls));
+        assert_eq!(pulls.last().map(|p| p.0), Some(1_000_000 * 48_000 / 44_100 - 1));
+        // a mixer returning too few channels and samples, and zero rates
+        let mut r = StreamResampler::new(0, 0);
+        let b = r.process(5, 64, |_, _| vec![vec![1.0; 3]]);
+        assert_eq!(b.len(), 1);
+        assert!(b[0].iter().all(|x| x.is_finite()));
+        assert!(r.process(0, 0, |_, _| Vec::new()).iter().all(Vec::is_empty));
+    }
+}
diff --git a/crates/codecs/src/audio.rs b/crates/codecs/src/audio.rs
index 7954473d..4942f6a6 100644
--- a/crates/codecs/src/audio.rs
+++ b/crates/codecs/src/audio.rs
@@ -15,8 +15,8 @@ use symphonia::core::probe::Hint;
 use crate::{CodecError, Result};
 
 enum Inner {
-    /// Our own AAC-LC decoder.
-    Aac { dec: Box<filmcraft_aac::Decoder>, asc: Vec<u8> },
+    /// Our own AAC-LC decoder. `up` interpolates HE-AAC's core to the output rate (see [`Upsample2x`]).
+    Aac { dec: Box<filmcraft_aac::Decoder>, asc: Vec<u8>, up: Option<Box<Upsample2x>> },
     /// AAC whose configuration arrives with the first frame (ADTS / LATM).
     AacPending,
     /// Our own Opus decoder (always 48 kHz output; pre-skip is left to container timestamps).
@@ -82,11 +82,13 @@ impl PacketDecoder {
         let dec = symphonia::default::get_codecs().make(&p, &DecoderOptions::default()).map_err(|e| CodecError::Unsupported(e.to_string()))?;
         Ok(Self { inner: Inner::Symphonia(dec), channels: 0 })
     }
-    /// AAC: our decoder for AAC-LC; symphonia for other object types (HE-AAC…).
+    /// AAC: our decoder for AAC-LC and the AAC-LC core of HE-AAC (v1/v2); symphonia for other
+    /// object types. `sample_rate` is the stream's output rate: when it is twice the core rate
+    /// (HE-AAC, see [`aac_output_rate`]) the decoded core is upsampled to it.
     pub fn aac(asc: &[u8], sample_rate: u32) -> Result<Self> {
-        let lc = asc.first().map(|b| b >> 3) == Some(2);
-        if lc && let Ok(dec) = filmcraft_aac::Decoder::new(asc) {
-            return Ok(Self { inner: Inner::Aac { dec: Box::new(dec), asc: asc.to_vec() }, channels: 0 });
+        if let Ok(dec) = filmcraft_aac::Decoder::new(asc) {
+            let up = (dec.sample_rate().checked_mul(2) == Some(sample_rate)).then(|| Box::new(Upsample2x::new(dec.channels())));
+            return Ok(Self { inner: Inner::Aac { dec: Box::new(dec), asc: asc.to_vec(), up }, channels: 0 });
         }
         Self::new(CODEC_TYPE_AAC, sample_rate, Some(asc.to_vec()))
     }
@@ -119,7 +121,7 @@ impl PacketDecoder {
             return Ok(());
         }
         let dec = filmcraft_aac::Decoder::new(asc).map_err(|e| CodecError::Unsupported(format!("AAC: {e}")))?;
-        self.inner = Inner::Aac { dec: Box::new(dec), asc: asc.to_vec() };
+        self.inner = Inner::Aac { dec: Box::new(dec), asc: asc.to_vec(), up: None };
         Ok(())
     }
     /// AC-3 (ATSC A/52).
@@ -133,7 +135,7 @@ impl PacketDecoder {
     pub fn for_isobmff(c: &filmcraft_isobmff::CodecConfig, rate: u32) -> Result<Self> {
         use filmcraft_isobmff::CodecConfig as C;
         match c {
-            C::Aac(a) => Self::aac(&a.asc, if a.sample_rate > 0 { a.sample_rate } else { rate }),
+            C::Aac(a) => Self::aac(&a.asc, if rate > 0 { rate } else { a.sample_rate }),
             C::Mp3 => Self::new(CODEC_TYPE_MP3, rate, None),
             C::Ac3 { .. } => Self::ac3(),
             C::Alac { cookie } => Self::new(CODEC_TYPE_ALAC, rate, Some(cookie.clone())),
@@ -145,8 +147,11 @@ impl PacketDecoder {
     /// Decode one packet into planar channels.
     pub fn decode(&mut self, data: &[u8], ts: u64) -> Result<Vec<Vec<f32>>> {
         let dec = match &mut self.inner {
-            Inner::Aac { dec, .. } => {
-                let out = dec.decode(data).map_err(|e| CodecError::Decode(e.to_string()))?;
+            Inner::Aac { dec, up, .. } => {
+                let mut out = dec.decode(data).map_err(|e| CodecError::Decode(e.to_string()))?;
+                if let Some(up) = up {
+                    out = up.process(out);
+                }
                 self.channels = out.len();
                 return Ok(out);
             }
@@ -202,10 +207,13 @@ impl PacketDecoder {
     }
     pub fn reset(&mut self) {
         match &mut self.inner {
-            Inner::Aac { dec, asc } => {
+            Inner::Aac { dec, asc, up } => {
                 if let Ok(d) = filmcraft_aac::Decoder::new(asc) {
                     **dec = d;
                 }
+                if let Some(up) = up {
+                    up.reset();
+                }
             }
             Inner::Opus { dec, .. } => dec.reset(),
             Inner::Symphonia(d) => d.reset(),
@@ -215,6 +223,94 @@ impl PacketDecoder {
     }
 }
 
+/// The output rate of an AAC stream: twice the core rate for HE-AAC (ISO/IEC 14496-3 §1.6.5).
+/// Explicit signalling names the rate in the `AudioSpecificConfig`; with implicit signalling (an
+/// AAC-LC config at 24 kHz or less) SBR is only visible as fill-element data in the access units,
+/// so the first ones are decoded to look for it. `None` if `asc` is not one our decoder reads.
+pub fn aac_output_rate<'a>(asc: &[u8], first_units: impl IntoIterator<Item = &'a [u8]>) -> Option<u32> {
+    let mut dec = filmcraft_aac::Decoder::new(asc).ok()?;
+    let core = dec.sample_rate();
+    if let Some(ext) = dec.config().extension_sample_rate {
+        return Some(ext);
+    }
+    if core <= 24_000 {
+        for au in first_units {
+            let _ = dec.decode(au);
+            if dec.sbr() {
+                return Some(core * 2);
+            }
+        }
+    }
+    Some(core)
+}
+
+/// Taps on each side of the interpolation point in [`Upsample2x`].
+const UPSAMPLE_HALF: usize = 12;
+
+/// 2× upsampler for HE-AAC. Our decoder reconstructs the AAC-LC core at half the output rate and
+/// not yet the SBR high band, so the core is interpolated to the output rate: timing and pitch are
+/// right and the band above the core's Nyquist stays empty. Even outputs are the input samples,
+/// odd ones a Blackman-windowed sinc half-band interpolation over 2 × [`UPSAMPLE_HALF`] inputs.
+/// The look-ahead delays the output by `UPSAMPLE_HALF` input samples (0.5 ms at 24 kHz); the
+/// state carries across packets, so a seek primed with the preceding packet gives the same
+/// samples as continuous decoding.
+#[derive(Clone)]
+pub struct Upsample2x {
+    taps: [f32; 2 * UPSAMPLE_HALF],
+    /// The last `2 × UPSAMPLE_HALF` input samples per channel.
+    hist: Vec<Vec<f32>>,
+}
+
+impl Upsample2x {
+    pub fn new(channels: usize) -> Self {
+        let m = UPSAMPLE_HALF as f64;
+        let mut taps = [0f32; 2 * UPSAMPLE_HALF];
+        let mut sum = 0.0;
+        for (k, t) in taps.iter_mut().enumerate() {
+            // distance from the interpolation point (halfway between two inputs)
+            let x = k as f64 - m + 0.5;
+            let sinc = (std::f64::consts::PI * x).sin() / (std::f64::consts::PI * x);
+            let w = 0.42 + 0.5 * (std::f64::consts::PI * x / m).cos() + 0.08 * (2.0 * std::f64::consts::PI * x / m).cos();
+            *t = (sinc * w) as f32;
+            sum += sinc * w;
+        }
+        for t in &mut taps {
+            *t = (*t as f64 / sum) as f32;
+        }
+        Self { taps, hist: vec![vec![0.0; 2 * UPSAMPLE_HALF]; channels] }
+    }
+
+    pub fn reset(&mut self) {
+        for h in &mut self.hist {
+            h.fill(0.0);
+        }
+    }
+
+    pub fn process(&mut self, input: Vec<Vec<f32>>) -> Vec<Vec<f32>> {
+        if self.hist.len() < input.len() {
+            self.hist.resize(input.len(), vec![0.0; 2 * UPSAMPLE_HALF]);
+        }
+        let m = UPSAMPLE_HALF;
+        input
+            .into_iter()
+            .zip(&mut self.hist)
+            .map(|(x, hist)| {
+                let mut buf = std::mem::take(hist);
+                buf.extend_from_slice(&x);
+                let mut out = Vec::with_capacity(x.len() * 2);
+                // output pair i is centred on buf[i + m]: the sample itself, then the point halfway
+                // to the next one from buf[i + 1 ..= i + 2m]
+                for w in buf.windows(2 * m + 1).take(x.len()) {
+                    out.push(w[m]);
+                    out.push(w[1..].iter().zip(&self.taps).map(|(a, b)| a * b).sum());
+                }
+                *hist = buf.split_off(buf.len() - 2 * m);
+                out
+            })
+            .collect()
+    }
+}
+
 /// Whether AC-3 audio decodes (our ATSC A/52 decoder).
 pub const AC3_DECODER: bool = true;
 
@@ -603,3 +699,43 @@ pub fn opener(name: &str, bytes: Arc<[u8]>) -> Option<std::result::Result<Shared
     }
     Some(AudioFileSource::decode(name, bytes).map(|s| Arc::new(s) as SharedSource).map_err(Into::into))
 }
+
+#[cfg(test)]
+mod he_aac_tests {
+    use super::*;
+
+    /// AAC-LC config, 22.05 kHz stereo.
+    const LC_22K: [u8; 2] = [0x13, 0x90];
+    /// An access unit holding only a fill element with SBR data (ID_FIL, EXT_SBR_DATA, ID_END).
+    const SBR_FILL: [u8; 3] = [0b1100_0011, 0b1010_0001, 0b1100_0000];
+
+    #[test]
+    fn output_rate_doubles_for_he_aac() {
+        // explicit: the extension rate from the config
+        assert_eq!(aac_output_rate(&[0x2B, 0x92, 0x08, 0x00], []), Some(44_100));
+        // implicit: SBR data in the first access units of a ≤ 24 kHz AAC-LC stream
+        assert_eq!(aac_output_rate(&LC_22K, [&SBR_FILL[..]]), Some(44_100));
+        assert_eq!(aac_output_rate(&LC_22K, []), Some(22_050));
+        // AAC-LC at 44.1 kHz stays as it is, even with SBR-looking fill data
+        assert_eq!(aac_output_rate(&[0x12, 0x10], [&SBR_FILL[..]]), Some(44_100));
+    }
+
+    #[test]
+    fn upsampler_interpolates_continuously_across_packets() {
+        let f = 1000.0 / 22_050.0;
+        let x: Vec<f32> = (0..4096).map(|i| (2.0 * std::f32::consts::PI * f * i as f32).sin()).collect();
+        let mut up = Upsample2x::new(1);
+        let mut y = Vec::new();
+        for chunk in x.chunks(1024) {
+            y.extend(up.process(vec![chunk.to_vec()]).remove(0));
+        }
+        assert_eq!(y.len(), 8192);
+        // output 2k + 1 is input k − UPSAMPLE_HALF + ½ (the look-ahead delay), within 0.1 %
+        let d = UPSAMPLE_HALF as f32;
+        for k in 100..4000 {
+            let want = (2.0 * std::f32::consts::PI * f * (k as f32 - d + 0.5)).sin();
+            assert!((y[2 * k + 1] - want).abs() < 1e-3, "{k}: {} vs {want}", y[2 * k + 1]);
+            assert_eq!(y[2 * k], x[k - UPSAMPLE_HALF]);
+        }
+    }
+}
diff --git a/crates/codecs/src/mp4.rs b/crates/codecs/src/mp4.rs
index 1ac8daca..d2149761 100644
--- a/crates/codecs/src/mp4.rs
+++ b/crates/codecs/src/mp4.rs
@@ -18,6 +18,9 @@ use crate::gop::{GopCache, VideoSamples};
 use crate::video::VideoDecoder;
 use crate::{CodecError, make_video_decoder};
 
+/// Most bytes read from one AAC access unit when probing for HE-AAC at open (8 channels × 6144 bits).
+const MAX_AAC_PROBE_UNIT: usize = 8 * 6144 / 8;
+
 struct AudioState {
     decoder: Option<PacketDecoder>,
     /// Decoded packets by sample index.
@@ -198,7 +201,21 @@ impl Mp4Source {
             let ap = entry.audio.clone().unwrap_or_default();
             let (rate, ch, bits) = match &entry.codec {
                 CodecConfig::Aac(a) => (
-                    if a.sample_rate > 0 { a.sample_rate } else { ap.sample_rate as u32 },
+                    {
+                        // HE-AAC plays at twice the core rate the AudioSpecificConfig starts with
+                        // an AAC access unit is at most 6144 bits per channel: a hostile `stsz` can't make us read more
+                        let units = t
+                            .samples
+                            .iter()
+                            .take(8)
+                            .filter_map(|x| filmcraft_media::reader::read_range(&*bytes.0, x.offset, (x.size as usize).min(MAX_AAC_PROBE_UNIT)).ok());
+                        let units: Vec<Vec<u8>> = units.collect();
+                        match crate::audio::aac_output_rate(&a.asc, units.iter().map(Vec::as_slice)) {
+                            Some(r) => r,
+                            None if a.sample_rate > 0 => a.sample_rate,
+                            None => ap.sample_rate as u32,
+                        }
+                    },
                     if a.channel_config > 0 { a.channel_config as u32 } else { ap.channels },
                     None,
                 ),
@@ -783,3 +800,40 @@ mod tests {
         }
     }
 }
+
+#[cfg(test)]
+mod he_aac_tests {
+    use filmcraft_isobmff::{Brand, Mp4Writer, SampleEntry, TrackConfig, WriteSample, WriterOptions};
+    use filmcraft_media::MediaSource;
+
+    /// #108: an HE-AAC MP4 (explicit SBR signalling, core 22.05 kHz, output 44.1 kHz, timescale
+    /// 44.1 kHz) built here from our AAC-LC encoder: the stream has no SBR data, which only the
+    /// high band would need. It was reported at the 22.05 kHz core rate, so sequences made from
+    /// such a clip were created at 22.05 kHz.
+    #[test]
+    fn he_aac_plays_at_the_output_rate() {
+        let core = 22_050u32;
+        let n = core as usize * 2;
+        let tone: Vec<f32> = (0..n).map(|i| 0.25 * (2.0 * std::f32::consts::PI * 1000.0 * i as f32 / core as f32).sin()).collect();
+        let mut enc = filmcraft_aac::Encoder::new(filmcraft_aac::EncoderConfig::cbr(core, 2, 64_000)).unwrap();
+        let mut aus = enc.encode(&[&tone, &tone]);
+        aus.extend(enc.flush());
+        // AOT 5, core 22.05 kHz, stereo, extension 44.1 kHz, core AOT 2
+        let asc = vec![0x2B, 0x92, 0x08, 0x00];
+        let mut mux = Mp4Writer::new(std::io::Cursor::new(Vec::new()), WriterOptions::new(Brand::Mp4)).unwrap();
+        let t = mux.add_track(TrackConfig::new(SampleEntry::aac(asc, 2, 44_100), 44_100)).unwrap();
+        for au in &aus {
+            mux.write_sample(t, WriteSample { data: au, duration: 2048, composition_offset: 0, is_sync: true }).unwrap();
+        }
+        let b = mux.finish().unwrap().into_inner();
+        let src = super::Mp4Source::open("he.mp4", b.into()).unwrap();
+        assert_eq!(src.info().audio.as_ref().unwrap().sample_rate, 44_100);
+        // one second from 0.5 s at 44.1 kHz: the 1 kHz tone at its level (not silence, not shifted)
+        let buf = src.audio(22_050, 44_100, 44_100).unwrap();
+        let x = &buf.channels[0];
+        let peak = x.iter().fold(0f32, |m, v| m.max(v.abs()));
+        assert!((0.2..0.3).contains(&peak), "peak {peak}");
+        let crossings = x.windows(2).filter(|w| w[0] < 0.0 && w[1] >= 0.0).count();
+        assert!((995..=1005).contains(&crossings), "{crossings} Hz");
+    }
+}
diff --git a/crates/engine/src/commands.rs b/crates/engine/src/commands.rs
index adbf45b5..3525f5fe 100644
--- a/crates/engine/src/commands.rs
+++ b/crates/engine/src/commands.rs
@@ -1374,22 +1374,15 @@ fn build() -> Vec<CommandSpec> {
             s.state.linked_selection = bool_p(p, "on").unwrap_or(!s.state.linked_selection);
             Ok(json!({"linkedSelection": s.state.linked_selection}))
         }),
-        cmd!("sequence.addTracks", "Add Tracks…", ["Sequence"], None, r#"{"video":n=1,"audio":n=0}"#, has_seq, |s, p| {
-            let nv = u64_p(p, "video").unwrap_or(1) as usize;
-            let na = u64_p(p, "audio").unwrap_or(0) as usize;
-            s.edit_sequence("Add Tracks", |q, ctx, _| {
-                for _ in 0..nv {
-                    let n = q.video_tracks.len() + 1;
-                    q.video_tracks.push(filmcraft_project::Track::new(TrackId(ctx.alloc()), TrackKind::Video, format!("Video {n}")));
-                }
-                for _ in 0..na {
-                    let n = q.audio_tracks.len() + 1;
-                    q.audio_tracks.push(filmcraft_project::Track::new(TrackId(ctx.alloc()), TrackKind::Audio, format!("Audio {n}")));
-                }
-                Ok(())
-            })?;
-            Ok(Value::Null)
-        }),
+        cmd!(
+            "sequence.addTracks",
+            "Add Tracks…",
+            ["Sequence"],
+            None,
+            r#"{"video":n=1,"audio":n=0,"submix":n=0,"videoAfter":"first"|"V2"|n?,"audioAfter":"first"|"A2"|n?,"submixAfter":"first"|"S1"|n?,"audioType":"standard|5.1|adaptive|mono"?,"submixType":"stereo|5.1|adaptive|mono"?}"#,
+            has_seq,
+            crate::sequence_tools::add_tracks
+        ),
         cmd!("sequence.deleteTrack", "Delete Track", [], None, r#"{"track":"V3"|id}"#, has_seq, |s, p| {
             let tr = track_p(s, p, "track", "sequence.deleteTrack")?.ok_or_else(|| bad("sequence.deleteTrack", "need `track`"))?;
             s.edit_sequence("Delete Track", |q, _, _| {
diff --git a/crates/engine/src/sequence_tools.rs b/crates/engine/src/sequence_tools.rs
index 86059131..d753b95e 100644
--- a/crates/engine/src/sequence_tools.rs
+++ b/crates/engine/src/sequence_tools.rs
@@ -27,7 +27,7 @@
 //! takes its own range and lands offset by the difference of the In points (J- and L-cuts).
 
 use filmcraft_edit as edit;
-use filmcraft_project::{ClipId, ItemId, Label, Marker, MarkerId, MarkerKind, SplitMarks, TrackId, TrackItem, TrackKind};
+use filmcraft_project::{AudioChannels, ClipId, ItemId, Label, Marker, MarkerId, MarkerKind, SplitMarks, TrackId, TrackItem, TrackKind};
 use filmcraft_time::{Tick, TimeRange};
 use serde_json::{Value, json};
 
@@ -448,6 +448,114 @@ fn make_subsequence(s: &mut Session, p: &Value) -> Result<Value> {
 
 /// Sequence ▸ Delete Tracks…: delete all empty video / audio tracks (`"empty"`) or one track per
 /// kind. A sequence always keeps at least one video and one audio track.
+/// The most tracks one `sequence.addTracks` adds of a kind, and the most a sequence may then
+/// have of it. (Amounts come from dialogs, scripts and the control channel: never trusted.)
+const MAX_ADDED_TRACKS: u64 = 99;
+const MAX_TRACKS: usize = 999;
+
+/// Where `sequence.addTracks` puts new tracks among the `len` tracks of their kind: the number of
+/// tracks before them. `key` holds `"first"` (Before First Track), a track of that kind by name
+/// (`"V2"`: after Video 2; `letter` is V, A or S) or a number (after that many tracks); without it
+/// they go after the last track.
+fn placement(p: &Value, key: &str, letter: char, len: usize) -> Result<usize> {
+    let cmd = "sequence.addTracks";
+    let after = match p.get(key) {
+        None | Some(Value::Null) => return Ok(len),
+        Some(v) => match (v.as_u64(), v.as_str()) {
+            (Some(n), _) => usize::try_from(n).ok(),
+            (None, Some(x)) if matches!(x.to_ascii_lowercase().as_str(), "first" | "before" | "beforefirst") => Some(0),
+            (None, Some(x)) if matches!(x.to_ascii_lowercase().as_str(), "last" | "end") => Some(len),
+            (None, Some(x)) => {
+                let mut rest = x.chars();
+                rest.next().filter(|c| c.eq_ignore_ascii_case(&letter)).and_then(|_| rest.as_str().parse::<usize>().ok())
+            }
+            (None, None) => None,
+        },
+    };
+    after.filter(|n| *n <= len).ok_or_else(|| bad(cmd, format!("`{key}` must be \"first\", a track ({letter}1–{letter}{len}) or how many tracks come before")))
+}
+
+/// Give the tracks that still carry a default name (`<prefix> <number>`) the number of their
+/// place, as Premiere numbers its tracks; a track the user named keeps its name.
+fn renumber(tracks: &mut [filmcraft_project::Track], prefix: &str) {
+    for (i, t) in tracks.iter_mut().enumerate() {
+        let default = t.name.strip_prefix(prefix).and_then(|r| r.strip_prefix(' ')).is_some_and(|n| !n.is_empty() && n.bytes().all(|b| b.is_ascii_digit()));
+        if default {
+            t.name = format!("{prefix} {}", i + 1);
+        }
+    }
+}
+
+/// Sequence ▸ Add Tracks…: `video` / `audio` / `submix` tracks, each kind at its own place
+/// (`videoAfter`, `audioAfter`, `submixAfter`; after the last track when not given), audio tracks
+/// of `audioType` (Standard unless given) and submix tracks of `submixType` (Stereo). One undo
+/// step. Tracks are numbered by their place, so default names after the new tracks move up.
+pub(crate) fn add_tracks(s: &mut Session, p: &Value) -> Result<Value> {
+    use crate::commands::u64_p;
+    let cmd = "sequence.addTracks";
+    let q = s.active_sequence().ok_or(EngineError::NoSequence)?;
+    let amount = |key: &str, default: u64, have: usize| -> Result<usize> {
+        let n = match p.get(key) {
+            None | Some(Value::Null) => default,
+            Some(_) => u64_p(p, key).ok_or_else(|| bad(cmd, format!("`{key}` must be a number of tracks")))?,
+        };
+        if n > MAX_ADDED_TRACKS {
+            return Err(bad(cmd, format!("`{key}`: at most {MAX_ADDED_TRACKS} tracks at a time")));
+        }
+        if have.saturating_add(n as usize) > MAX_TRACKS {
+            return Err(bad(cmd, format!("`{key}`: a sequence has at most {MAX_TRACKS} tracks of a kind")));
+        }
+        Ok(n as usize)
+    };
+    let (nv, na, ns) = (amount("video", 1, q.video_tracks.len())?, amount("audio", 0, q.audio_tracks.len())?, amount("submix", 0, q.submix_tracks.len())?);
+    let (at_v, at_a, at_s) = (
+        placement(p, "videoAfter", 'V', q.video_tracks.len())?,
+        placement(p, "audioAfter", 'A', q.audio_tracks.len())?,
+        placement(p, "submixAfter", 'S', q.submix_tracks.len())?,
+    );
+    let channels = |key: &str, default: AudioChannels| match p.get(key).and_then(Value::as_str) {
+        None => Ok(default),
+        Some(x) => crate::mixer::channels_from(x).ok_or_else(|| bad(cmd, format!("`{key}`: unknown track type {x:?} (standard, stereo, 5.1, adaptive, mono)"))),
+    };
+    let (audio_type, submix_type) = (channels("audioType", AudioChannels::Stereo)?, channels("submixType", AudioChannels::Stereo)?);
+    if nv + na + ns == 0 {
+        return Err(bad(cmd, "no tracks to add"));
+    }
+    let added = s.edit_sequence("Add Tracks", |q, ctx, _| {
+        let mut new = |kind: TrackKind, prefix: &str, channels: Option<AudioChannels>| {
+            let mut t = filmcraft_project::Track::new(TrackId(ctx.alloc()), kind, format!("{prefix} 0"));
+            if let Some(c) = channels {
+                t.channels = c;
+            }
+            t
+        };
+        let mut ids: [Vec<u64>; 3] = Default::default();
+        for (k, (tracks, n, at, kind, prefix, ch)) in [
+            (&mut q.video_tracks, nv, at_v, TrackKind::Video, "Video", None),
+            (&mut q.audio_tracks, na, at_a, TrackKind::Audio, "Audio", Some(audio_type)),
+            (&mut q.submix_tracks, ns, at_s, TrackKind::Audio, "Submix", Some(submix_type)),
+        ]
+        .into_iter()
+        .enumerate()
+        {
+            let at = at.min(tracks.len());
+            for i in 0..n {
+                let t = new(kind, prefix, ch);
+                if let Some(slot) = ids.get_mut(k) {
+                    slot.push(t.id.0);
+                }
+                tracks.insert(at + i, t);
+            }
+            if n > 0 {
+                renumber(tracks, prefix);
+            }
+        }
+        Ok(ids)
+    })?;
+    let [video, audio, submix] = added;
+    Ok(json!({"video": video, "audio": audio, "submix": submix}))
+}
+
 fn delete_tracks(s: &mut Session, p: &Value) -> Result<Value> {
     let seq_id = s.state.active_sequence.ok_or(EngineError::NoSequence)?;
     let q = s.active_sequence().ok_or(EngineError::NoSequence)?;
diff --git a/crates/engine/src/sequence_tools_tests.rs b/crates/engine/src/sequence_tools_tests.rs
index c69016a4..d9df881f 100644
--- a/crates/engine/src/sequence_tools_tests.rs
+++ b/crates/engine/src/sequence_tools_tests.rs
@@ -1,7 +1,7 @@
 //! Sequence / Markers menu commands (`sequence_tools`).
 
 use super::*;
-use filmcraft_project::{Label, MarkerKind};
+use filmcraft_project::{AudioChannels, Label, MarkerKind, TrackKind};
 use serde_json::json;
 
 fn demo() -> Session {
@@ -189,6 +189,108 @@ fn make_subsequence_from_in_out_and_from_selection() {
     assert!(s.project.items.values().all(|i| i.name != "Shot 3"));
 }
 
+/// Sequence ▸ Add Tracks…: where the new tracks go, what they are, and what the amounts may be.
+/// Premiere Pro 26.5.2: Placement is "Before First Track" or after one of the tracks (after the
+/// last by default); a track added after Video 1 is the new Video 2, and what was on Video 2 is
+/// then on Video 3; audio tracks are Standard, 5.1, Adaptive or Mono, submix tracks Stereo, 5.1,
+/// Adaptive or Mono.
+#[test]
+fn add_tracks_places_and_types_them() {
+    let mut s = demo();
+    let names = |s: &Session, kind: TrackKind| s.active_sequence().unwrap().tracks(kind).iter().map(|t| t.name.clone()).collect::<Vec<_>>();
+    let before = s.active_sequence().unwrap().clone();
+    assert_eq!(names(&s, TrackKind::Video), ["Video 1", "Video 2", "Video 3"]);
+    let on_v2 = before.video_tracks[1].items[0].id;
+
+    // without a place: after the last track (and one video track when nothing is said, as before)
+    let r = s.execute("sequence.addTracks", json!({})).unwrap();
+    assert_eq!((r["video"].as_array().unwrap().len(), r["audio"].as_array().unwrap().len(), r["submix"].as_array().unwrap().len()), (1, 0, 0));
+    assert_eq!(names(&s, TrackKind::Video), ["Video 1", "Video 2", "Video 3", "Video 4"]);
+    assert_eq!(s.active_sequence().unwrap().video_tracks[3].id.0, r["video"][0].as_u64().unwrap());
+    s.execute("edit.undo", json!({})).unwrap();
+
+    // after Video 1: two new empty tracks there, the tracks above move up and are numbered again
+    s.execute("sequence.addTracks", json!({"video": 2, "videoAfter": "V1"})).unwrap();
+    let q = s.active_sequence().unwrap();
+    assert_eq!(names(&s, TrackKind::Video), ["Video 1", "Video 2", "Video 3", "Video 4", "Video 5"]);
+    assert!(q.video_tracks[1].items.is_empty() && q.video_tracks[2].items.is_empty());
+    assert_eq!(q.video_tracks[3].id, before.video_tracks[1].id);
+    assert_eq!(q.video_tracks[3].items[0].id, on_v2, "what was on Video 2 is on Video 4");
+    assert_eq!(q.audio_tracks.len(), before.audio_tracks.len(), "no audio track was asked for");
+    // one undo step
+    s.execute("edit.undo", json!({})).unwrap();
+    assert_eq!(*s.active_sequence().unwrap(), before);
+
+    // before the first track; a name the user gave is kept, default names follow their place
+    s.edit_sequence("Rename Track", |q, _, _| {
+        q.audio_tracks[1].name = "Music".into();
+        Ok(())
+    })
+    .unwrap();
+    s.execute("sequence.addTracks", json!({"video": 0, "audio": 1, "audioAfter": "first", "audioType": "mono"})).unwrap();
+    assert_eq!(names(&s, TrackKind::Audio), ["Audio 1", "Audio 2", "Music", "Audio 4"]);
+    let q = s.active_sequence().unwrap();
+    assert_eq!(q.audio_tracks[0].channels, AudioChannels::Mono);
+    assert_eq!(q.audio_tracks[1].id, before.audio_tracks[0].id);
+    // a number counts the tracks before the new ones; "Standard" is a stereo track
+    for (ty, want) in [("standard", AudioChannels::Stereo), ("5.1", AudioChannels::Surround51), ("adaptive", AudioChannels::Adaptive)] {
+        let r = s.execute("sequence.addTracks", json!({"video": 0, "audio": 1, "audioAfter": 2, "audioType": ty})).unwrap();
+        let q = s.active_sequence().unwrap();
+        assert_eq!((q.audio_tracks[2].id.0, q.audio_tracks[2].channels), (r["audio"][0].as_u64().unwrap(), want), "{ty}");
+    }
+
+    // submix tracks: stereo unless said, placed among the submix tracks
+    assert!(before.submix_tracks.is_empty());
+    s.execute("sequence.addTracks", json!({"video": 0, "submix": 2})).unwrap();
+    s.execute("sequence.addTracks", json!({"video": 0, "submix": 1, "submixAfter": "S1", "submixType": "5.1"})).unwrap();
+    let q = s.active_sequence().unwrap();
+    assert_eq!(
+        q.submix_tracks.iter().map(|t| (t.name.as_str(), t.channels)).collect::<Vec<_>>(),
+        [("Submix 1", AudioChannels::Stereo), ("Submix 2", AudioChannels::Surround51), ("Submix 3", AudioChannels::Stereo)]
+    );
+    // all three kinds at once are one undo step
+    let before = s.active_sequence().unwrap().clone();
+    let r = s.execute("sequence.addTracks", json!({"video": 1, "audio": 1, "submix": 1})).unwrap();
+    assert_eq!((r["video"].as_array().unwrap().len(), r["audio"].as_array().unwrap().len(), r["submix"].as_array().unwrap().len()), (1, 1, 1));
+    s.execute("edit.undo", json!({})).unwrap();
+    assert_eq!(*s.active_sequence().unwrap(), before);
+}
+
+/// Amounts and places come from a dialog, a script or the control channel: a wrong one is an
+/// error that changes nothing, and no amount can make the sequence grow without bound.
+#[test]
+fn add_tracks_refuses_what_it_cannot_do() {
+    let mut s = demo();
+    let before = s.active_sequence().unwrap().clone();
+    for bad in [
+        json!({"video": 0}),
+        json!({"video": 100}),
+        json!({"video": u64::MAX}),
+        json!({"video": 0, "audio": 1_000_000}),
+        json!({"video": "many"}),
+        json!({"video": 1, "videoAfter": "V9"}),
+        json!({"video": 1, "videoAfter": "A1"}),
+        json!({"video": 1, "videoAfter": 4}),
+        json!({"video": 1, "videoAfter": "top"}),
+        json!({"video": 0, "audio": 1, "audioAfter": "V1"}),
+        json!({"video": 0, "audio": 1, "audioType": "quad"}),
+        json!({"video": 0, "submix": 1, "submixAfter": "S1"}),
+        json!({"video": 0, "submix": 1, "submixType": "standard-ish"}),
+    ] {
+        let r = s.execute("sequence.addTracks", bad.clone());
+        assert!(r.is_err(), "{bad} was accepted: {r:?}");
+        assert_eq!(*s.active_sequence().unwrap(), before, "{bad} changed the sequence");
+    }
+    // 99 at a time, and a sequence stops at 999 tracks of a kind
+    for _ in 0..10 {
+        s.execute("sequence.addTracks", json!({"video": 99})).unwrap();
+    }
+    assert_eq!(s.active_sequence().unwrap().video_tracks.len(), 3 + 990);
+    assert!(s.execute("sequence.addTracks", json!({"video": 7})).is_err());
+    s.execute("sequence.addTracks", json!({"video": 6})).unwrap();
+    assert_eq!(s.active_sequence().unwrap().video_tracks.len(), 999);
+}
+
 #[test]
 fn delete_tracks_empty_and_specific() {
     let mut s = demo();
diff --git a/crates/export/src/lib.rs b/crates/export/src/lib.rs
index d07bd349..d50e1f30 100644
--- a/crates/export/src/lib.rs
+++ b/crates/export/src/lib.rs
@@ -628,7 +628,10 @@ impl AudioEncoder for AacEncoder {
 }
 
 fn aac_factory(_format: Format, sample_rate: u32, channels: u32, s: &ExportSettings) -> Option<Result<Box<dyn AudioEncoder>>> {
-    let bps = s.audio.bitrate_kbps.clamp(32, 512) * 1000;
+    // AAC caps a frame at 6144 bits per channel (ISO/IEC 14496-3 §4.5.3.2): 6 bits per sample, so
+    // 264.6 kbps for stereo at 22.05 kHz; a higher setting is capped instead of refused
+    let max = (6 * sample_rate as u64 * channels.max(1) as u64).min(u32::MAX as u64) as u32;
+    let bps = (s.audio.bitrate_kbps.clamp(32, 512) * 1000).min(max);
     Some(
         filmcraft_aac::Encoder::new(filmcraft_aac::EncoderConfig::cbr(sample_rate, channels as usize, bps))
             .map(|enc| Box::new(AacEncoder { enc, rate: sample_rate, channels }) as Box<dyn AudioEncoder>)
diff --git a/crates/export/src/settings_tests.rs b/crates/export/src/settings_tests.rs
index 9904d3c3..4a7562fe 100644
--- a/crates/export/src/settings_tests.rs
+++ b/crates/export/src/settings_tests.rs
@@ -398,3 +398,15 @@ fn builtin_presets_are_valid_and_unique() {
     let back: ExportPreset = serde_json::from_value(json).unwrap();
     assert_eq!(back.settings.adaptive_bitrate, Some(0.2));
 }
+
+/// #108: the default 320 kbps is more than AAC can carry in stereo at 22.05 kHz (6 bits per sample
+/// and channel). The export failed with "bitrate out of range"; it is now capped to the limit.
+#[test]
+fn aac_bitrate_is_capped_for_low_sample_rates() {
+    let mut s = ExportSettings::default();
+    s.audio.bitrate_kbps = 320;
+    for (rate, ch) in [(22_050, 2), (16_000, 2), (8_000, 6), (48_000, 2)] {
+        let enc = aac_factory(Format::H264, rate, ch, &s).expect("AAC is always available");
+        assert!(enc.is_ok(), "{rate} Hz × {ch}: {:?}", enc.err());
+    }
+}
diff --git a/crates/ui-egui/examples/bench_audio.rs b/crates/ui-egui/examples/bench_audio.rs
new file mode 100644
index 00000000..5bf1fbfd
--- /dev/null
+++ b/crates/ui-egui/examples/bench_audio.rs
@@ -0,0 +1,205 @@
+//! Headless audio playback bench: plays a project's sequences through a simulated 48 kHz stereo
+//! device with a 512-frame buffer (the app's defaults) and counts dropouts, the way the device
+//! would hear them. Nothing is played out loud.
+//!
+//! ```sh
+//! FILMCRAFT_DATA_DIR=/tmp/fc-bench cargo run --release -p filmcraft-ui-egui --example bench_audio -- \
+//!     "Project.fcproj" --seconds 20 [--sequence "One hero" …] [--mode direct|ahead|both] [--json out.json] [--wav dir]
+//! ```
+//!
+//! `direct` mixes inside the device callback (how playback worked before sound was mixed ahead);
+//! `ahead` plays through [`play_ahead`] as the app does now. A callback that returns after its
+//! buffer was due is a dropout, and the device cycles it overran are lost. For `ahead` the ring's
+//! own underruns (silence played because the mixer fell behind) count too. `--wav` writes what a
+//! device would have played for each run, to listen to.
+//!
+//! The simulated device thread runs at normal priority (CoreAudio's own I/O thread runs higher),
+//! so absolute counts are pessimistic for both modes; compare the modes on the same machine load.
+
+use std::sync::Arc;
+use std::sync::atomic::Ordering;
+use std::time::{Duration, Instant};
+
+use filmcraft_engine::Session;
+use filmcraft_project::ItemId;
+use filmcraft_ui_egui::play_ahead::{self, AudioStats};
+use serde_json::{Value, json};
+
+const SR: u32 = 48_000;
+const PERIOD: usize = 512;
+const CH: usize = 2;
+
+#[derive(Default)]
+struct Run {
+    callbacks: u64,
+    /// Callbacks that returned after their buffer was due.
+    late: u64,
+    /// Callbacks whose own work took longer than one buffer: a dropout on any device.
+    overruns: u64,
+    /// Device cycles lost to late callbacks.
+    lost_cycles: u64,
+    max_callback_ms: f64,
+    /// `ahead` only: callbacks the ring could not fill.
+    ring_underruns: u64,
+    min_lead_ms: Option<f64>,
+    /// What a device whose I/O thread always wakes on time plays (interleaved stereo): a callback
+    /// that overran loses its buffer and the slots it overran to silence.
+    heard: Vec<f32>,
+}
+
+impl Run {
+    /// What a device whose I/O thread always wakes on time would hear: overrunning callbacks plus
+    /// buffers the ring could not fill. (`late` also counts this bench's normal-priority device
+    /// thread waking late under load, which CoreAudio's real-time thread does not.)
+    fn dropouts(&self) -> u64 {
+        self.overruns + self.ring_underruns
+    }
+}
+
+/// Calls `fill` once per device period for `seconds` of wall time, as a device does.
+fn device(fill: &mut dyn FnMut(&mut [f32], usize), seconds: f64) -> Run {
+    let period = PERIOD as f64 / SR as f64;
+    let total = (seconds / period) as u64;
+    let mut buf = vec![0.0f32; PERIOD * CH];
+    let mut r = Run::default();
+    let t0 = Instant::now();
+    let mut k = 0u64;
+    while k < total {
+        let due = Duration::from_secs_f64(period * k as f64);
+        if let Some(wait) = due.checked_sub(t0.elapsed()) {
+            std::thread::sleep(wait);
+        }
+        let c0 = Instant::now();
+        fill(&mut buf, CH);
+        r.callbacks += 1;
+        let took = c0.elapsed().as_secs_f64();
+        r.max_callback_ms = r.max_callback_ms.max(took * 1e3);
+        if took > period {
+            r.overruns += 1;
+            let lost = (took / period).ceil() as usize;
+            r.heard.resize(r.heard.len() + lost * PERIOD * CH, 0.0);
+        } else {
+            r.heard.extend_from_slice(&buf);
+        }
+        let done = t0.elapsed().as_secs_f64();
+        let deadline = period * (k + 1) as f64;
+        if done > deadline {
+            r.late += 1;
+            let next = (done / period).ceil() as u64;
+            r.lost_cycles += next.saturating_sub(k + 1);
+            k = next;
+        } else {
+            k += 1;
+        }
+    }
+    r
+}
+
+/// 32-bit float stereo WAV at the device rate.
+fn write_wav(path: &std::path::Path, samples: &[f32]) -> std::io::Result<()> {
+    let data = (samples.len() * 4) as u32;
+    let mut b = Vec::with_capacity(44 + data as usize);
+    b.extend_from_slice(b"RIFF");
+    b.extend_from_slice(&(36 + data).to_le_bytes());
+    b.extend_from_slice(b"WAVEfmt ");
+    b.extend_from_slice(&16u32.to_le_bytes());
+    b.extend_from_slice(&3u16.to_le_bytes()); // IEEE float
+    b.extend_from_slice(&(CH as u16).to_le_bytes());
+    b.extend_from_slice(&SR.to_le_bytes());
+    b.extend_from_slice(&(SR * CH as u32 * 4).to_le_bytes());
+    b.extend_from_slice(&(CH as u16 * 4).to_le_bytes());
+    b.extend_from_slice(&32u16.to_le_bytes());
+    b.extend_from_slice(b"data");
+    b.extend_from_slice(&data.to_le_bytes());
+    for x in samples {
+        b.extend_from_slice(&x.to_le_bytes());
+    }
+    std::fs::write(path, b)
+}
+
+fn play(s: &Session, seq: ItemId, mode: &str, seconds: f64) -> Run {
+    let mut mix = filmcraft_ui_egui::playback_mix(s, seq, SR);
+    match mode {
+        "direct" => {
+            let mut cursor = 0i64;
+            let mut fill = move |buf: &mut [f32], ch: usize| {
+                mix(cursor, buf, ch);
+                cursor += (buf.len() / ch) as i64;
+            };
+            device(&mut fill, seconds)
+        }
+        _ => {
+            let stats = Arc::new(AudioStats::default());
+            let mut fill = play_ahead::spawn(mix, 0, SR, CH, stats.clone());
+            let mut r = device(&mut *fill, seconds);
+            r.ring_underruns = stats.underruns.load(Ordering::Relaxed);
+            let min = stats.min_lead_frames.load(Ordering::Relaxed);
+            r.min_lead_ms = (min != u64::MAX).then(|| min as f64 * 1e3 / SR as f64);
+            r
+        }
+    }
+}
+
+fn main() {
+    filmcraft_platform::register();
+    let mut args = std::env::args().skip(1);
+    let (mut project, mut seconds, mut only, mut mode, mut json_out, mut wav_dir) = (None, 20.0, Vec::new(), "both".to_string(), None, None);
+    while let Some(a) = args.next() {
+        match a.as_str() {
+            "--seconds" => seconds = args.next().and_then(|v| v.parse().ok()).unwrap_or(seconds),
+            "--sequence" => only.extend(args.next()),
+            "--mode" => mode = args.next().unwrap_or(mode),
+            "--json" => json_out = args.next(),
+            "--wav" => wav_dir = args.next().map(std::path::PathBuf::from),
+            p => project = Some(p.to_string()),
+        }
+    }
+    let Some(project) = project else {
+        eprintln!("usage: bench_audio <project.fcproj> [--seconds N] [--sequence NAME] [--mode direct|ahead|both] [--json PATH] [--wav DIR]");
+        std::process::exit(2);
+    };
+    let mut s = Session::default();
+    if let Err(e) = s.execute("file.open", json!({ "path": project })) {
+        eprintln!("{project}: {e}");
+        std::process::exit(1);
+    }
+    let mut seqs: Vec<(ItemId, String)> = s.project.sequences().map(|i| (i.id, i.name.clone())).filter(|(_, n)| only.is_empty() || only.contains(n)).collect();
+    seqs.sort_by(|a, b| a.1.cmp(&b.1));
+    let modes: Vec<&str> = if mode == "both" { vec!["direct", "ahead"] } else { vec![mode.as_str()] };
+    println!(
+        "{:<48} {:<7} {:>9} {:>8} {:>9} {:>9} {:>8} {:>11} {:>9}",
+        "sequence", "mode", "callbacks", "dropouts", "overruns", "underruns", "late", "max cb (ms)", "min lead"
+    );
+    let mut out = Vec::<Value>::new();
+    for (id, name) in &seqs {
+        for m in &modes {
+            let r = play(&s, *id, m, seconds);
+            if let Some(d) = &wav_dir
+                && let Err(e) = write_wav(&d.join(format!("{name} - {m}.wav")), &r.heard)
+            {
+                eprintln!("{}: {e}", d.display());
+            }
+            let lost_ms = r.lost_cycles as f64 * PERIOD as f64 * 1e3 / SR as f64;
+            let lead = r.min_lead_ms.map_or("-".to_string(), |v| format!("{v:.0} ms"));
+            println!(
+                "{:<48} {:<7} {:>9} {:>8} {:>9} {:>9} {:>8} {:>11.1} {:>9}",
+                name,
+                m,
+                r.callbacks,
+                r.dropouts(),
+                r.overruns,
+                r.ring_underruns,
+                r.late,
+                r.max_callback_ms,
+                lead
+            );
+            out.push(json!({"sequence": name, "mode": m, "seconds": seconds, "callbacks": r.callbacks, "dropouts": r.dropouts(), "overruns": r.overruns, "late": r.late,
+                "lostMs": lost_ms, "ringUnderruns": r.ring_underruns, "maxCallbackMs": r.max_callback_ms, "minLeadMs": r.min_lead_ms}));
+        }
+    }
+    if let Some(p) = json_out
+        && let Err(e) = std::fs::write(&p, serde_json::to_string_pretty(&out).unwrap_or_default())
+    {
+        eprintln!("{p}: {e}");
+    }
+}
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 98dd0463..cb923eba 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -19,6 +19,8 @@ pub mod links;
 pub mod menus;
 pub mod panels;
 pub mod perf;
+#[cfg(not(target_arch = "wasm32"))]
+pub mod play_ahead;
 pub mod state;
 pub mod theme;
 pub mod widgets;
@@ -45,6 +47,10 @@ pub trait AudioOut {
     fn stop(&mut self);
     /// Device sample rate.
     fn sample_rate(&self) -> u32;
+    /// Device output channels (what `fill` will be called with).
+    fn channels(&self) -> usize {
+        2
+    }
     /// Frames played since `start` (the playback master clock), if the device reports it.
     fn played_frames(&self) -> Option<u64>;
     /// Hosts and devices that can be chosen in Settings ▸ Audio Hardware.
@@ -106,6 +112,8 @@ pub enum Dialog {
     AudioGain,
     /// Sequence ▸ Delete Tracks….
     DeleteTracks,
+    /// Sequence ▸ Add Tracks….
+    AddTracks,
 }
 
 #[derive(Default)]
@@ -118,6 +126,9 @@ pub struct Playback {
     anchor_tick: Tick,
     /// Audio frames played at anchor (when the audio clock drives).
     pub audio_clock: bool,
+    /// Audio underruns for the current (or last) play (desktop: sound is mixed ahead).
+    #[cfg(not(target_arch = "wasm32"))]
+    pub audio_stats: std::sync::Arc<play_ahead::AudioStats>,
     /// Shown / dropped frame accounting for the current (or last) play.
     pub meter: frames::PlaybackMeter,
     /// Waiting for the first frames before starting the clock: when the wait began (egui time,
@@ -592,57 +603,24 @@ impl FilmcraftApp {
             return;
         }
         let Some(seq_id) = self.session.state.active_sequence else { return };
-        let project = self.session.project.clone();
-        let provider = self.session.media.provider(project.clone(), self.session.services.clone());
-        let previews = self.session.previews.clone();
         let start_tick = self.session.playhead();
-        // Settings ▸ Audio Hardware ▸ Output Mapping
-        let map = [self.session.prefs.audio_hardware.map_left, self.session.prefs.audio_hardware.map_right];
-        // Preferences ▸ Audio ▸ 5.1 Mixdown Type: how a 5.1 Mix plays on a stereo device
-        let mixdown = filmcraft_audio_dsp::channels::Mixdown::from_id(&self.session.prefs.audio.mixdown_type).unwrap_or_default();
         let Some(a) = self.audio.as_mut() else { return };
         let sr = a.sample_rate();
-        let mut cursor = start_tick.to_units_floor(sr as i64);
-        let cues = panels::voiceover::cues(&self.session, sr);
-        previews.live.publish_project(project.clone());
-        let fill = Box::new(move |buf: &mut [f32], ch: usize| {
-            // the newest project snapshot: mixer moves and other edits are heard while playing
-            let project = previews.live.project().filter(|p| p.sequence(seq_id).is_some()).unwrap_or_else(|| project.clone());
-            let Some(seq) = project.sequence(seq_id) else { return };
-            let n = buf.len() / ch.max(1);
-            // Mix at the sequence rate; convert when the device rate differs (nearest sample).
-            let seq_sr = seq.settings.sample_rate;
-            // a 5.1 Mix plays as six channels (L, R, C, LFE, Ls, Rs) on a device with at least six
-            use filmcraft_audio_dsp::channels::Layout;
-            let layout = if ch >= 6 && seq.settings.audio_master == filmcraft_project::AudioChannels::Surround51 { Layout::Surround51 } else { Layout::Stereo };
-            let mix = if seq_sr == sr {
-                previews.mix_layout(&project, seq_id, cursor, n, &provider, layout, mixdown)
-            } else {
-                let s0 = (cursor as i128 * seq_sr as i128 / sr as i128) as i64;
-                let m = n * seq_sr as usize / sr as usize + 2;
-                let b = previews.mix_layout(&project, seq_id, s0, m, &provider, layout, mixdown);
-                let mut out = filmcraft_frame::AudioBuffer::silence(sr, b.channels.len(), n);
-                for (o, c) in out.channels.iter_mut().zip(&b.channels) {
-                    for (i, x) in o.iter_mut().enumerate() {
-                        let j = (i * seq_sr as usize / sr as usize).min(m - 1);
-                        *x = c[j];
-                    }
-                }
-                out
-            };
-            if layout == Layout::Surround51 {
-                buf.fill(0.0);
-                for (i, frame) in buf.chunks_mut(ch).enumerate() {
-                    for (c, x) in frame.iter_mut().take(6).enumerate() {
-                        *x = mix.channels[c][i];
-                    }
-                }
-            } else {
-                filmcraft_engine::settings::map_output(&mix.channels[0], &mix.channels[1.min(mix.channels.len() - 1)], buf, ch, map);
-            }
-            panels::voiceover::mix_cues(buf, ch, cursor, &cues);
-            cursor += n as i64;
-        });
+        let cursor = start_tick.to_units_floor(sr as i64);
+        let mix = playback_mix(&self.session, seq_id, sr);
+        // the old stream stops before the new mixer resets the underrun counters
+        a.stop();
+        // Desktop: mix ahead on a thread so the device callback never waits on decoding.
+        #[cfg(not(target_arch = "wasm32"))]
+        let fill = play_ahead::spawn(mix, cursor, sr, a.channels(), self.playback.audio_stats.clone());
+        #[cfg(target_arch = "wasm32")]
+        let fill = {
+            let (mut mix, mut cursor) = (mix, cursor);
+            Box::new(move |buf: &mut [f32], ch: usize| {
+                mix(cursor, buf, ch);
+                cursor += (buf.len() / ch.max(1)) as i64;
+            })
+        };
         match a.start(fill) {
             Ok(_) => self.playback.audio_clock = true,
             Err(e) => {
@@ -1352,6 +1330,62 @@ impl eframe::App for FilmcraftApp {
     }
 }
 
+/// The program mix that playback plays: `mix(device_frame, interleaved, channels)` renders the
+/// active sequence at device rate `sr` from `device_frame`, with the live project snapshot (edits
+/// made while playing are heard), Output Mapping, the 5.1 mixdown and voice-over cues.
+pub fn playback_mix(session: &Session, seq_id: filmcraft_project::ItemId, sr: u32) -> impl FnMut(i64, &mut [f32], usize) + Send + 'static {
+    let project = session.project.clone();
+    let provider = session.media.provider(project.clone(), session.services.clone());
+    let previews = session.previews.clone();
+    // Settings ▸ Audio Hardware ▸ Output Mapping
+    let map = [session.prefs.audio_hardware.map_left, session.prefs.audio_hardware.map_right];
+    // Preferences ▸ Audio ▸ 5.1 Mixdown Type: how a 5.1 Mix plays on a stereo device
+    let mixdown = filmcraft_audio_dsp::channels::Mixdown::from_id(&session.prefs.audio.mixdown_type).unwrap_or_default();
+    let cues = panels::voiceover::cues(session, sr);
+    previews.live.publish_project(project.clone());
+    let mut resampler = None;
+    move |cursor: i64, buf: &mut [f32], ch: usize| {
+        // the newest project snapshot: mixer moves and other edits are heard while playing
+        let project = previews.live.project().filter(|p| p.sequence(seq_id).is_some()).unwrap_or_else(|| project.clone());
+        let Some(seq) = project.sequence(seq_id) else {
+            buf.fill(0.0);
+            return;
+        };
+        let n = buf.len() / ch.max(1);
+        let seq_sr = seq.settings.sample_rate;
+        // a 5.1 Mix plays as six channels (L, R, C, LFE, Ls, Rs) on a device with at least six
+        use filmcraft_audio_dsp::channels::Layout;
+        let layout = if ch >= 6 && seq.settings.audio_master == filmcraft_project::AudioChannels::Surround51 { Layout::Surround51 } else { Layout::Stereo };
+        let mix = if seq_sr == sr {
+            previews.mix_layout(&project, seq_id, cursor, n, &provider, layout, mixdown)
+        } else {
+            // Mix at the sequence rate as one continuous stream, interpolated to the device rate
+            // (started afresh when the rate or the channel layout changes while playing).
+            let key = (seq_sr, layout);
+            if resampler.as_ref().is_none_or(|(k, _)| *k != key) {
+                resampler = Some((key, filmcraft_audio_dsp::resample::StreamResampler::new(seq_sr, sr)));
+            }
+            let Some((_, r)) = resampler.as_mut() else {
+                buf.fill(0.0);
+                return;
+            };
+            let channels = r.process(cursor, n, |s0, m| previews.mix_layout(&project, seq_id, s0, m, &provider, layout, mixdown).channels);
+            filmcraft_frame::AudioBuffer { sample_rate: sr, channels }
+        };
+        if layout == Layout::Surround51 {
+            buf.fill(0.0);
+            for (i, frame) in buf.chunks_mut(ch).enumerate() {
+                for (c, x) in frame.iter_mut().take(6).enumerate() {
+                    *x = mix.channels[c][i];
+                }
+            }
+        } else {
+            filmcraft_engine::settings::map_output(&mix.channels[0], &mix.channels[1.min(mix.channels.len() - 1)], buf, ch, map);
+        }
+        panels::voiceover::mix_cues(buf, ch, cursor, &cues);
+    }
+}
+
 #[cfg(test)]
 mod gpu_fallback_tests {
     use filmcraft_render::plan::{FramePlan, PlanLayer};
diff --git a/crates/ui-egui/src/menus.rs b/crates/ui-egui/src/menus.rs
index dff937cf..620aaea4 100644
--- a/crates/ui-egui/src/menus.rs
+++ b/crates/ui-egui/src/menus.rs
@@ -327,6 +327,21 @@ pub fn invoke(app: &mut FilmcraftApp, ctx: &egui::Context, id: &str, params: Val
             filmcraft_engine::find_command(id).map_or(Ok(()), |c| (c.enabled)(&app.session))?;
             return crate::panels::remix::open(app, ctx);
         }
+        // Add Tracks… from the menu opens the dialog (new tracks after the last ones, as Premiere
+        // offers); with params it adds the tracks directly.
+        "sequence.addTracks" if params.as_object().is_none_or(|m| m.is_empty()) => {
+            filmcraft_engine::find_command(id).map_or(Ok(()), |c| (c.enabled)(&app.session))?;
+            let seq = app.session.active_sequence();
+            let count = |f: fn(&filmcraft_engine::project::Sequence) -> usize| seq.map_or(0, f);
+            app.ui.add_tracks = crate::state::AddTracksDraft {
+                video_after: count(|q| q.video_tracks.len()),
+                audio_after: count(|q| q.audio_tracks.len()),
+                submix_after: count(|q| q.submix_tracks.len()),
+                ..Default::default()
+            };
+            app.dialog = Some(crate::Dialog::AddTracks);
+            return Ok(json!({"dialog": "addTracks"}));
+        }
         "sequence.deleteTracks" if params.as_object().is_none_or(|m| m.is_empty()) => {
             filmcraft_engine::find_command(id).map_or(Ok(()), |c| (c.enabled)(&app.session))?;
             app.ui.delete_tracks = Default::default();
diff --git a/crates/ui-egui/src/panels/dialogs.rs b/crates/ui-egui/src/panels/dialogs.rs
index 3032b53d..f3390701 100644
--- a/crates/ui-egui/src/panels/dialogs.rs
+++ b/crates/ui-egui/src/panels/dialogs.rs
@@ -58,6 +58,12 @@ pub fn show(app: &mut FilmcraftApp, ctx: &egui::Context) {
             }
             return;
         }
+        Dialog::AddTracks => {
+            if !add_tracks(app, ctx) {
+                app.dialog = None;
+            }
+            return;
+        }
         Dialog::NewSequence | Dialog::Preferences | Dialog::Recovery | Dialog::RevertConfirm => {}
     }
     if !open {
@@ -146,6 +152,134 @@ fn audio_gain(app: &mut FilmcraftApp, ctx: &egui::Context) -> bool {
     keep
 }
 
+/// Sequence ▸ Add Tracks…, laid out as Premiere Pro's dialog: "Add video tracks" (Amount,
+/// Placement), "Add audio tracks" (Amount, Placement, Track type: Standard, 5.1, Adaptive, Mono)
+/// and "Add audio submix tracks" (Amount, Placement, Track type: Stereo, 5.1, Adaptive, Mono).
+/// Placement is "Before First Track" or after one of the tracks; the submix placement is off
+/// while the sequence has no submix track. Returns whether the dialog stays open.
+///
+/// Automation ids, with `<kind>` = `video`, `audio` or `submix`: `addTracks.<kind>.amount`,
+/// `addTracks.<kind>.placement` and, while its list is open, `addTracks.<kind>.placement.option.<n>`
+/// (n tracks before the new ones; 0 = Before First Track); `addTracks.<audio|submix>.type` and
+/// `addTracks.<audio|submix>.type.option.<standard|stereo|5.1|adaptive|mono>`; `addTracks.ok`,
+/// `addTracks.cancel`.
+fn add_tracks(app: &mut FilmcraftApp, ctx: &egui::Context) -> bool {
+    let Some(seq) = app.session.active_sequence() else { return false };
+    let names = |tracks: &[filmcraft_engine::project::Track]| tracks.iter().map(|t| t.name.clone()).collect::<Vec<_>>();
+    let (vnames, anames, snames) = (names(&seq.video_tracks), names(&seq.audio_tracks), names(&seq.submix_tracks));
+    let mut d = app.ui.add_tracks.clone();
+    let mut keep = true;
+    let mut apply = false;
+    let mut elems: Vec<(String, egui::Rect, String)> = Vec::new();
+    const AUDIO_TYPES: [(&str, &str); 4] = [("standard", "Standard"), ("5.1", "5.1"), ("adaptive", "Adaptive"), ("mono", "Mono")];
+    const SUBMIX_TYPES: [(&str, &str); 4] = [("stereo", "Stereo"), ("5.1", "5.1"), ("adaptive", "Adaptive"), ("mono", "Mono")];
+    egui::Window::new("Add Tracks").collapsible(false).resizable(false).default_width(320.0).anchor(egui::Align2::CENTER_CENTER, [0.0, 0.0]).show(ctx, |ui| {
+        // everything spans the width the window has: no empty band beside the fields
+        ui.set_width(ui.available_width());
+        // the label column is as wide as its widest label; the lists take the rest of the row,
+        // up to the group's right edge (as in Premiere's dialog)
+        let font = egui::TextStyle::Body.resolve(ui.style());
+        let label_w = ["Amount", "Placement", "Track type"]
+            .iter()
+            .map(|l| ui.painter().layout_no_wrap(l.to_string(), font.clone(), egui::Color32::WHITE).size().x)
+            .fold(0.0, f32::max);
+        let row_h = ui.spacing().interact_size.y;
+        let label = |ui: &mut egui::Ui, text: &str| {
+            ui.allocate_ui_with_layout(egui::vec2(label_w, row_h), egui::Layout::right_to_left(egui::Align::Center), |ui| ui.label(text));
+        };
+        let groups: [(&str, &str, &mut u32, &mut usize, &[String], Option<(&mut String, &[(&str, &str); 4])>); 3] = [
+            ("video", "Add video tracks", &mut d.video, &mut d.video_after, &vnames, None),
+            ("audio", "Add audio tracks", &mut d.audio, &mut d.audio_after, &anames, Some((&mut d.audio_type, &AUDIO_TYPES))),
+            ("submix", "Add audio submix tracks", &mut d.submix, &mut d.submix_after, &snames, Some((&mut d.submix_type, &SUBMIX_TYPES))),
+        ];
+        for (kind, title, amount, after, tracks, track_type) in groups {
+            ui.add_space(4.0);
+            ui.label(egui::RichText::new(title).strong());
+            ui.group(|ui| {
+                ui.set_width(ui.available_width());
+                ui.horizontal(|ui| {
+                    label(ui, "Amount");
+                    let r = ui.add(egui::DragValue::new(amount).range(0..=99).speed(0.1));
+                    elems.push((format!("addTracks.{kind}.amount"), r.rect, amount.to_string()));
+                });
+                ui.horizontal(|ui| {
+                    label(ui, "Placement");
+                    *after = (*after).min(tracks.len());
+                    let place = |n: usize| match n.checked_sub(1).and_then(|i| tracks.get(i)) {
+                        Some(name) => format!("After {name}"),
+                        None => "Before First Track".to_string(),
+                    };
+                    let shown = place(*after);
+                    let list_w = ui.available_width();
+                    // nowhere to choose from while the sequence has no track of the kind
+                    ui.add_enabled_ui(!tracks.is_empty(), |ui| {
+                        let r = egui::ComboBox::from_id_salt(("add-tracks-placement", kind)).selected_text(&shown).width(list_w).show_ui(ui, |ui| {
+                            for n in 0..=tracks.len() {
+                                let label = place(n);
+                                let o = ui.selectable_value(after, n, &label);
+                                elems.push((format!("addTracks.{kind}.placement.option.{n}"), o.rect, label));
+                            }
+                        });
+                        elems.push((format!("addTracks.{kind}.placement"), r.response.rect, shown.clone()));
+                    });
+                });
+                if let Some((chosen, types)) = track_type {
+                    ui.horizontal(|ui| {
+                        label(ui, "Track type");
+                        let shown = types.iter().find(|(id, _)| *id == chosen.as_str()).map_or(types[0].1, |(_, label)| label).to_string();
+                        let list_w = ui.available_width();
+                        let r = egui::ComboBox::from_id_salt(("add-tracks-type", kind)).selected_text(&shown).width(list_w).show_ui(ui, |ui| {
+                            for (id, label) in types {
+                                let o = ui.selectable_value(chosen, id.to_string(), *label);
+                                elems.push((format!("addTracks.{kind}.type.option.{id}"), o.rect, label.to_string()));
+                            }
+                        });
+                        elems.push((format!("addTracks.{kind}.type"), r.response.rect, shown));
+                    });
+                }
+            });
+        }
+        ui.add_space(10.0);
+        ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+            let o = ui.add(egui::Button::new(egui::RichText::new("OK").color(egui::Color32::WHITE)).fill(app.tokens.accent));
+            elems.push(("addTracks.ok".into(), o.rect, "OK".into()));
+            if o.clicked() {
+                apply = true;
+            }
+            let c = ui.button("Cancel");
+            elems.push(("addTracks.cancel".into(), c.rect, "Cancel".into()));
+            if c.clicked() {
+                keep = false;
+            }
+        });
+    });
+    for (id, r, l) in elems {
+        app.auto.add(&id, r, &l);
+    }
+    if ctx.input(|i| i.key_pressed(egui::Key::Escape)) {
+        keep = false;
+    }
+    if ctx.input(|i| i.key_pressed(egui::Key::Enter)) && !ctx.egui_wants_keyboard_input() {
+        apply = true;
+    }
+    if apply {
+        // nothing asked for: OK closes the dialog and changes nothing
+        if d.video > 0 || d.audio > 0 || d.submix > 0 {
+            let params = serde_json::json!({
+                "video": d.video, "videoAfter": d.video_after,
+                "audio": d.audio, "audioAfter": d.audio_after, "audioType": d.audio_type,
+                "submix": d.submix, "submixAfter": d.submix_after, "submixType": d.submix_type,
+            });
+            if let Err(e) = app.session.execute("sequence.addTracks", params) {
+                app.ui.status = e.to_string();
+            }
+        }
+        keep = false;
+    }
+    app.ui.add_tracks = d;
+    keep
+}
+
 /// Sequence ▸ Delete Tracks…: "Delete Video Tracks" / "Delete Audio Tracks" checkboxes, each with a
 /// track choice (All Empty Tracks or one track). Returns whether the dialog stays open.
 ///
diff --git a/crates/ui-egui/src/perf.rs b/crates/ui-egui/src/perf.rs
index 6164faa0..79df3c52 100644
--- a/crates/ui-egui/src/perf.rs
+++ b/crates/ui-egui/src/perf.rs
@@ -20,6 +20,11 @@ pub fn stats(app: &FilmcraftApp) -> Value {
         "resolution": app.ui.program.res.label(),
         "draftDecode": app.session.prefs.playback.draft_decode,
     });
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        let sr = app.audio.as_ref().map_or(0, |a| a.sample_rate());
+        v["playback"]["audio"] = app.playback.audio_stats.to_json(sr);
+    }
     let mut frames = app.frames.stats().to_json();
     frames["workers"] = json!(app.frames.workers());
     frames["queued"] = json!(app.frames.queue_len());
diff --git a/crates/ui-egui/src/play_ahead.rs b/crates/ui-egui/src/play_ahead.rs
new file mode 100644
index 00000000..b3243cbd
--- /dev/null
+++ b/crates/ui-egui/src/play_ahead.rs
@@ -0,0 +1,193 @@
+//! Audio played ahead: playback mixes on its own thread into a ring buffer that stays
+//! [`LEAD_S`] ahead of the device, and the device callback only copies samples out of it.
+//!
+//! Mixing (decoding AAC, effects, opening a clip's file at a cut) used to run inside the device
+//! callback, which has one I/O buffer (~10 ms) to return. On a busy Mac any stall there missed the
+//! device's deadline, heard as static and dropouts that were different on every play. Now a stall
+//! only drains part of the lead; the callback never decodes, allocates or takes a lock.
+//!
+//! When the ring does run dry (an underrun), the callback plays silence for the missing frames and
+//! counts them, and the mixer skips the same span, so sound stays locked to the picture.
+//!
+//! Desktop only: the web build has no threads and keeps mixing in its callback.
+
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, AtomicU32, AtomicU64, Ordering};
+use std::time::Duration;
+
+/// How far ahead of the device the mixer keeps the ring (s). Edits made while playing are heard
+/// this much later.
+pub const LEAD_S: f64 = 0.2;
+/// Frames mixed per block on the mixing thread.
+pub const BLOCK_FRAMES: usize = 1024;
+/// Ceiling on mixed channels (large interfaces and aggregate devices stay well under it).
+const MAX_CHANNELS: usize = 64;
+
+/// Underrun counters for the current (or last) play, readable from any thread (`perf.stats`).
+#[derive(Debug, Default)]
+pub struct AudioStats {
+    /// Device callbacks served.
+    pub callbacks: AtomicU64,
+    /// Callbacks the ring could not fill completely.
+    pub underruns: AtomicU64,
+    /// Frames played as silence because the ring was empty.
+    pub missing_frames: AtomicU64,
+    /// Fewest frames left in the ring after a callback, once it first filled to the lead
+    /// (`u64::MAX` until then).
+    pub min_lead_frames: AtomicU64,
+}
+
+impl AudioStats {
+    pub fn reset(&self) {
+        self.callbacks.store(0, Ordering::Relaxed);
+        self.underruns.store(0, Ordering::Relaxed);
+        self.missing_frames.store(0, Ordering::Relaxed);
+        self.min_lead_frames.store(u64::MAX, Ordering::Relaxed);
+    }
+
+    pub fn to_json(&self, sample_rate: u32) -> serde_json::Value {
+        let min = self.min_lead_frames.load(Ordering::Relaxed);
+        serde_json::json!({
+            "callbacks": self.callbacks.load(Ordering::Relaxed),
+            "underruns": self.underruns.load(Ordering::Relaxed),
+            "missingFrames": self.missing_frames.load(Ordering::Relaxed),
+            "minLeadMs": if min == u64::MAX || sample_rate == 0 { serde_json::Value::Null } else { serde_json::json!(min as f64 * 1e3 / sample_rate as f64) },
+        })
+    }
+}
+
+/// Single-producer, single-consumer ring of interleaved f32 samples (stored as bits, so it needs
+/// no `unsafe`). Indices count samples written / read since the start and only grow.
+struct Ring {
+    slots: Vec<AtomicU32>,
+    write: AtomicU64,
+    read: AtomicU64,
+}
+
+impl Ring {
+    fn new(samples: usize) -> Self {
+        Self { slots: (0..samples.max(1)).map(|_| AtomicU32::new(0)).collect(), write: AtomicU64::new(0), read: AtomicU64::new(0) }
+    }
+
+    fn len(&self) -> usize {
+        self.write.load(Ordering::Acquire).saturating_sub(self.read.load(Ordering::Acquire)) as usize
+    }
+
+    /// Producer: append as many samples as fit; returns how many were written.
+    fn push(&self, src: &[f32]) -> usize {
+        let cap = self.slots.len();
+        let w = self.write.load(Ordering::Relaxed);
+        let free = cap.saturating_sub(w.saturating_sub(self.read.load(Ordering::Acquire)) as usize);
+        let n = src.len().min(free);
+        for (i, x) in src.iter().take(n).enumerate() {
+            if let Some(s) = self.slots.get(((w + i as u64) % cap as u64) as usize) {
+                s.store(x.to_bits(), Ordering::Relaxed);
+            }
+        }
+        self.write.store(w + n as u64, Ordering::Release);
+        n
+    }
+
+    /// Consumer: fill `dst` (`dev_ch` interleaved channels) with whole frames of `ch` channels from
+    /// the ring: the first `min(ch, dev_ch)` channels copied, any further device channels zeroed.
+    /// Returns the frames copied; the rest of `dst` is left for the caller.
+    fn pop(&self, dst: &mut [f32], dev_ch: usize, ch: usize) -> usize {
+        let cap = self.slots.len();
+        let r = self.read.load(Ordering::Relaxed);
+        let avail = self.write.load(Ordering::Acquire).saturating_sub(r) as usize / ch.max(1);
+        let n = (dst.len() / dev_ch.max(1)).min(avail);
+        for (f, frame) in dst.chunks_mut(dev_ch.max(1)).take(n).enumerate() {
+            for (c, x) in frame.iter_mut().enumerate() {
+                *x = if c < ch {
+                    self.slots.get(((r + (f * ch + c) as u64) % cap as u64) as usize).map_or(0.0, |s| f32::from_bits(s.load(Ordering::Relaxed)))
+                } else {
+                    0.0
+                };
+            }
+        }
+        self.read.store(r + (n * ch) as u64, Ordering::Release);
+        n
+    }
+}
+
+/// Stops the mixing thread when the device callback that owns it is dropped (stream stopped).
+struct StopOnDrop(Arc<AtomicBool>);
+
+impl Drop for StopOnDrop {
+    fn drop(&mut self) {
+        self.0.store(true, Ordering::Release);
+    }
+}
+
+/// Mixes `mix(cursor, buf, channels)` (interleaved frames from device frame `cursor`) on its own
+/// thread, from `start`, and returns the device callback that plays it (`fill(buf, channels)`).
+/// The first block is mixed before this returns, so playback starts with sound. The thread runs
+/// until the returned callback is dropped.
+pub fn spawn<M>(mut mix: M, start: i64, sample_rate: u32, channels: usize, stats: Arc<AudioStats>) -> Box<dyn FnMut(&mut [f32], usize) + Send>
+where
+    M: FnMut(i64, &mut [f32], usize) + Send + 'static,
+{
+    stats.reset();
+    let ch = channels.clamp(1, MAX_CHANNELS);
+    let lead = ((sample_rate as f64 * LEAD_S) as usize).max(BLOCK_FRAMES * 2);
+    let ring = Arc::new(Ring::new(lead * ch));
+    let mut block = vec![0.0; BLOCK_FRAMES * ch];
+    mix(start, &mut block, ch);
+    ring.push(&block);
+    // Frames the callback played as silence that the mixer has not skipped yet.
+    let skip = Arc::new(AtomicU64::new(0));
+    let stop = Arc::new(AtomicBool::new(false));
+    {
+        let (ring, skip, stop) = (ring.clone(), skip.clone(), stop.clone());
+        let body = move || {
+            let mut cursor = start + BLOCK_FRAMES as i64;
+            while !stop.load(Ordering::Acquire) {
+                if ring.len() + block.len() > lead * ch {
+                    std::thread::sleep(Duration::from_millis(2));
+                    continue;
+                }
+                cursor += skip.swap(0, Ordering::AcqRel) as i64;
+                block.fill(0.0);
+                mix(cursor, &mut block, ch);
+                cursor += BLOCK_FRAMES as i64;
+                // silence played while this block was mixing: its first frames are already late
+                let late = skip.swap(0, Ordering::AcqRel) as usize;
+                let drop = late.min(BLOCK_FRAMES);
+                cursor += (late - drop) as i64;
+                ring.push(block.get(drop * ch..).unwrap_or_default());
+            }
+        };
+        let spawned = std::thread::Builder::new().name("filmcraft-audio-mix".into()).spawn(move || {
+            if std::panic::catch_unwind(std::panic::AssertUnwindSafe(body)).is_err() {
+                log::error!("audio mixing stopped after a panic; playback continues silent");
+            }
+        });
+        if let Err(e) = spawned {
+            log::error!("audio mixing thread: {e}");
+        }
+    }
+    let guard = StopOnDrop(stop);
+    let mut primed = false;
+    Box::new(move |buf: &mut [f32], dev_ch: usize| {
+        let _keep = &guard;
+        stats.callbacks.fetch_add(1, Ordering::Relaxed);
+        // a device wider or narrower than the mix (it changed since `channels` was read) still
+        // plays the channels both have
+        let dev_ch = dev_ch.max(1);
+        let have = ring.pop(buf, dev_ch, ch);
+        if let Some(rest) = buf.get_mut(have * dev_ch..) {
+            rest.fill(0.0);
+        }
+        let n = buf.len() / dev_ch;
+        if have < n {
+            stats.underruns.fetch_add(1, Ordering::Relaxed);
+            stats.missing_frames.fetch_add((n - have) as u64, Ordering::Relaxed);
+            skip.fetch_add((n - have) as u64, Ordering::AcqRel);
+        }
+        let left = ring.len();
+        primed |= left + 2 * BLOCK_FRAMES * ch > lead * ch;
+        if primed {
+            stats.min_lead_frames.fetch_min((left / ch) as u64, Ordering::Relaxed);
+        }
+    })
+}
diff --git a/crates/ui-egui/src/state.rs b/crates/ui-egui/src/state.rs
index 04c29090..12f810e2 100644
--- a/crates/ui-egui/src/state.rs
+++ b/crates/ui-egui/src/state.rs
@@ -422,6 +422,9 @@ pub struct UiState {
     /// Audio Gain dialog draft (mode, dB values).
     #[serde(default)]
     pub audio_gain: AudioGainDraft,
+    /// Add Tracks dialog draft.
+    #[serde(default)]
+    pub add_tracks: AddTracksDraft,
     /// Delete Tracks dialog draft.
     #[serde(default)]
     pub delete_tracks: DeleteTracksDraft,
@@ -701,6 +704,30 @@ pub struct AudioGainDraft {
     pub all_peaks_db: f64,
 }
 
+/// The Add Tracks dialog (Sequence ▸ Add Tracks…): for video, audio and audio submix tracks, how
+/// many to add and where (`*_after`: how many tracks of the kind come before the new ones; 0 is
+/// "Before First Track"), and the type of the audio and submix tracks (`standard` / `stereo`,
+/// `5.1`, `adaptive`, `mono`). Premiere's defaults: one video track and one audio track after the
+/// last ones, no submix track.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(default)]
+pub struct AddTracksDraft {
+    pub video: u32,
+    pub video_after: usize,
+    pub audio: u32,
+    pub audio_after: usize,
+    pub audio_type: String,
+    pub submix: u32,
+    pub submix_after: usize,
+    pub submix_type: String,
+}
+
+impl Default for AddTracksDraft {
+    fn default() -> Self {
+        Self { video: 1, video_after: 0, audio: 1, audio_after: 0, audio_type: "standard".into(), submix: 0, submix_after: 0, submix_type: "stereo".into() }
+    }
+}
+
 /// The Delete Tracks dialog (Sequence ▸ Delete Tracks…): per kind, whether to delete and which
 /// track (`"empty"` = All Empty Tracks, or a track name such as `"V2"`).
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -770,6 +797,7 @@ impl Default for UiState {
             mixer_meter_input_only: false,
             settings: None,
             audio_gain: AudioGainDraft::default(),
+            add_tracks: AddTracksDraft::default(),
             delete_tracks: DeleteTracksDraft::default(),
             gfx_edit: None,
             pen_points: vec![],
diff --git a/crates/ui-egui/tests/play_ahead.rs b/crates/ui-egui/tests/play_ahead.rs
new file mode 100644
index 00000000..f14a04d9
--- /dev/null
+++ b/crates/ui-egui/tests/play_ahead.rs
@@ -0,0 +1,128 @@
+//! Audio mixed ahead (`play_ahead`): driven by a wall-clock device. Its own test binary, because one
+//! test panics a mixer on purpose and the crash hook's tests read the process-wide last panic.
+
+use std::sync::Arc;
+use std::sync::atomic::{AtomicU64, Ordering};
+use std::time::{Duration, Instant};
+
+use filmcraft_ui_egui::play_ahead::{AudioStats, BLOCK_FRAMES, LEAD_S, spawn};
+
+const SR: u32 = 48_000;
+const PERIOD: usize = 512;
+
+/// A mixer whose every sample is its own device frame number, so the output shows which frame
+/// played where.
+fn frame_numbers(delay: Duration, calls: Arc<AtomicU64>) -> impl FnMut(i64, &mut [f32], usize) + Send + 'static {
+    move |cursor, buf, ch| {
+        calls.fetch_add(1, Ordering::Relaxed);
+        std::thread::sleep(delay);
+        for (i, f) in buf.chunks_mut(ch).enumerate() {
+            f.fill((cursor + i as i64) as f32);
+        }
+    }
+}
+
+/// Plays `callbacks` device periods, one every period on the wall clock as a device does, and
+/// returns the interleaved output.
+fn play(fill: &mut Box<dyn FnMut(&mut [f32], usize) + Send>, ch: usize, callbacks: usize) -> Vec<f32> {
+    let period = Duration::from_secs_f64(PERIOD as f64 / SR as f64);
+    let t0 = Instant::now();
+    let mut out = Vec::new();
+    for k in 0..callbacks {
+        let mut buf = vec![f32::NAN; PERIOD * ch];
+        fill(&mut buf, ch);
+        out.extend_from_slice(&buf);
+        if let Some(wait) = (period * (k as u32 + 1)).checked_sub(t0.elapsed()) {
+            std::thread::sleep(wait);
+        }
+    }
+    out
+}
+
+#[test]
+fn plays_every_frame_in_order_from_the_start() {
+    let stats = Arc::new(AudioStats::default());
+    let mut fill = spawn(frame_numbers(Duration::ZERO, Arc::default()), 1000, SR, 2, stats.clone());
+    std::thread::sleep(Duration::from_millis(30));
+    let out = play(&mut fill, 2, 60);
+    for (p, f) in out.chunks(2).enumerate() {
+        assert_eq!(f, [(1000 + p) as f32; 2], "frame {p}");
+    }
+    assert_eq!(stats.underruns.load(Ordering::Relaxed), 0);
+    assert_eq!(stats.callbacks.load(Ordering::Relaxed), 60);
+}
+
+#[test]
+fn a_mixer_slower_than_the_device_plays_silence_and_stays_in_time() {
+    // each 1024-frame block takes 40 ms to mix, about twice its 21 ms of sound
+    let stats = Arc::new(AudioStats::default());
+    let mut fill = spawn(frame_numbers(Duration::from_millis(40), Arc::default()), 1, SR, 2, stats.clone());
+    let out = play(&mut fill, 2, 120);
+    assert!(stats.underruns.load(Ordering::Relaxed) > 0);
+    assert!(stats.missing_frames.load(Ordering::Relaxed) > 0);
+    let mut heard = 0;
+    for (p, f) in out.chunks(2).enumerate() {
+        assert!(f.iter().all(|x| x.is_finite()), "frame {p} left unwritten");
+        if f[0] != 0.0 {
+            // every frame that plays is the one due at that moment: no drift after an underrun
+            assert_eq!(f[0], (1 + p) as f32, "frame {p}");
+            heard += 1;
+        }
+    }
+    assert!(heard > PERIOD, "some sound still plays");
+}
+
+#[test]
+fn a_mixer_that_panics_leaves_silence_not_a_crash() {
+    let stats = Arc::new(AudioStats::default());
+    let mut first = true;
+    let mix = move |cursor: i64, buf: &mut [f32], ch: usize| {
+        if !first {
+            panic!("mixer bug");
+        }
+        first = false;
+        frame_numbers(Duration::ZERO, Arc::default())(cursor, buf, ch);
+    };
+    let mut fill = spawn(mix, 1, SR, 2, stats.clone());
+    let out = play(&mut fill, 2, 20);
+    assert!(out.iter().all(|x| x.is_finite()));
+    assert_eq!(out.get(..2), Some(&[1.0, 1.0][..]), "the block mixed before the panic plays");
+    assert!(stats.underruns.load(Ordering::Relaxed) > 0);
+}
+
+#[test]
+fn a_device_with_other_channels_plays_the_channels_both_have() {
+    for dev_ch in [1, 6, 10, 16] {
+        let stats = Arc::new(AudioStats::default());
+        let mut fill = spawn(frame_numbers(Duration::ZERO, Arc::default()), 1, SR, 2, stats.clone());
+        // let the mixer get ahead first, as a device would before its first callback
+        std::thread::sleep(Duration::from_millis(30));
+        let out = play(&mut fill, dev_ch, 4);
+        for (p, f) in out.chunks(dev_ch).enumerate() {
+            let want: Vec<f32> = (0..dev_ch).map(|c| if c < 2 { (1 + p) as f32 } else { 0.0 }).collect();
+            assert_eq!(f, &want[..], "{dev_ch} channels, frame {p}");
+        }
+        assert_eq!(stats.underruns.load(Ordering::Relaxed), 0, "{dev_ch} channels");
+    }
+}
+
+#[test]
+fn stopping_the_stream_stops_the_mixer() {
+    let calls = Arc::new(AtomicU64::new(0));
+    let fill = spawn(frame_numbers(Duration::ZERO, calls.clone()), 1, SR, 2, Arc::default());
+    std::thread::sleep(Duration::from_millis(50));
+    drop(fill);
+    std::thread::sleep(Duration::from_millis(20));
+    let after_stop = calls.load(Ordering::Relaxed);
+    std::thread::sleep(Duration::from_millis(50));
+    assert_eq!(calls.load(Ordering::Relaxed), after_stop);
+}
+
+#[test]
+fn the_ring_keeps_no_more_than_the_lead() {
+    let calls = Arc::new(AtomicU64::new(0));
+    let _fill = spawn(frame_numbers(Duration::ZERO, calls.clone()), 1, SR, 2, Arc::default());
+    std::thread::sleep(Duration::from_millis(100));
+    let lead_blocks = (SR as f64 * LEAD_S / BLOCK_FRAMES as f64).ceil() as u64;
+    assert!(calls.load(Ordering::Relaxed) <= lead_blocks + 1, "mixed {} blocks", calls.load(Ordering::Relaxed));
+}
diff --git a/crates/ui-egui/tests/sequence_ui.rs b/crates/ui-egui/tests/sequence_ui.rs
index 3e498217..ad1b300e 100644
--- a/crates/ui-egui/tests/sequence_ui.rs
+++ b/crates/ui-egui/tests/sequence_ui.rs
@@ -1,4 +1,4 @@
-//! Headless UI tests of the Sequence / Markers menu additions: the Delete Tracks dialog, the
+//! Headless UI tests of the Sequence / Markers menu additions: the Add Tracks and Delete Tracks dialogs, the
 //! through-edit marks on the timeline, the Markers panel colour filter and the Shift+; gap key.
 //! The real `FilmcraftApp` under `egui_kittest`, driven over the control channel by automation id.
 
@@ -64,12 +64,161 @@ impl Driver {
         self.frames(2);
     }
 
+    fn app(&mut self) -> &mut FilmcraftApp {
+        self.harness.state_mut()
+    }
+
+    /// (video, audio, submix) track names of the active sequence, lowest first.
+    fn tracks(&mut self) -> [Vec<String>; 3] {
+        let q = self.app().session.active_sequence().unwrap();
+        [&q.video_tracks, &q.audio_tracks, &q.submix_tracks].map(|t| t.iter().map(|t| t.name.clone()).collect())
+    }
+
+    fn label(&mut self, id: &str) -> String {
+        let v = self.ok("ui.elements", json!({"prefix": id}));
+        let e = v.as_array().unwrap().iter().find(|e| e["id"] == json!(id)).unwrap_or_else(|| panic!("no element {id}")).clone();
+        e["label"].as_str().unwrap_or_default().to_string()
+    }
+
     fn ids(&mut self, prefix: &str) -> Vec<String> {
         let v = self.ok("ui.elements", json!({"prefix": prefix}));
         v.as_array().unwrap().iter().filter_map(|e| e["id"].as_str().map(str::to_string)).collect()
     }
 }
 
+/// Sequence ▸ Add Tracks… opens Premiere Pro's dialog instead of adding a video track at once:
+/// amounts, a placement and (for audio and submix tracks) a track type for each kind, with
+/// Premiere's defaults (one video and one audio track after the last ones, no submix track).
+#[test]
+fn add_tracks_dialog_offers_amount_placement_and_type() {
+    let mut d = Driver::demo();
+    let before = d.tracks();
+    assert_eq!(
+        before,
+        [vec!["Video 1", "Video 2", "Video 3"], vec!["Audio 1", "Audio 2", "Audio 3"], vec![]]
+            .map(|v: Vec<&str>| v.into_iter().map(String::from).collect::<Vec<_>>())
+    );
+    let r = d.ok("ui.menu.invoke", json!({"id": "sequence.addTracks"}));
+    assert_eq!(r["dialog"], "addTracks", "{r}");
+    d.frames(3);
+    assert_eq!(d.tracks(), before, "opening the dialog adds nothing");
+    let ids = d.ids("addTracks.");
+    for id in [
+        "addTracks.video.amount",
+        "addTracks.video.placement",
+        "addTracks.audio.amount",
+        "addTracks.audio.placement",
+        "addTracks.audio.type",
+        "addTracks.submix.amount",
+        "addTracks.submix.placement",
+        "addTracks.submix.type",
+        "addTracks.ok",
+        "addTracks.cancel",
+    ] {
+        assert!(ids.iter().any(|i| i == id), "{id} missing: {ids:?}");
+    }
+    assert!(!ids.iter().any(|i| i == "addTracks.video.type"), "video tracks have no type");
+    // the defaults, as shown
+    for (id, shown) in [
+        ("addTracks.video.amount", "1"),
+        ("addTracks.video.placement", "After Video 3"),
+        ("addTracks.audio.amount", "1"),
+        ("addTracks.audio.placement", "After Audio 3"),
+        ("addTracks.audio.type", "Standard"),
+        ("addTracks.submix.amount", "0"),
+        ("addTracks.submix.placement", "Before First Track"),
+        ("addTracks.submix.type", "Stereo"),
+    ] {
+        assert_eq!(d.label(id), shown, "{id}");
+    }
+    // OK with the defaults: one video and one audio track after the last ones
+    d.click("addTracks.ok");
+    assert!(d.ids("addTracks.").is_empty(), "dialog closed");
+    let [v, a, s] = d.tracks();
+    assert_eq!((v.len(), a.len(), s.len()), (4, 4, 0));
+    assert_eq!((v[3].as_str(), a[3].as_str()), ("Video 4", "Audio 4"));
+    assert_eq!(d.app().session.active_sequence().unwrap().audio_tracks[3].channels, filmcraft_engine::project::AudioChannels::Stereo);
+    // one undo step for the whole dialog
+    d.exec("edit.undo", json!({}));
+    assert_eq!(d.tracks(), before);
+}
+
+#[test]
+fn add_tracks_dialog_places_and_types_the_new_tracks() {
+    let mut d = Driver::demo();
+    let on_v2 = d.app().session.active_sequence().unwrap().video_tracks[1].items[0].id;
+    d.ok("ui.menu.invoke", json!({"id": "sequence.addTracks"}));
+    d.frames(3);
+    // Placement lists "Before First Track" and every track; pick "After Video 1"
+    d.click("addTracks.video.placement");
+    let options = d.ids("addTracks.video.placement.option.");
+    assert_eq!(options.len(), 4, "{options:?}");
+    assert_eq!(
+        (d.label("addTracks.video.placement.option.0"), d.label("addTracks.video.placement.option.1")),
+        ("Before First Track".into(), "After Video 1".into())
+    );
+    d.click("addTracks.video.placement.option.1");
+    assert_eq!(d.label("addTracks.video.placement"), "After Video 1");
+    // audio: a mono track before the first one
+    d.click("addTracks.audio.placement");
+    d.click("addTracks.audio.placement.option.0");
+    d.click("addTracks.audio.type");
+    let types = d.ids("addTracks.audio.type.option.");
+    assert_eq!(types.len(), 4, "Standard, 5.1, Adaptive, Mono: {types:?}");
+    d.click("addTracks.audio.type.option.mono");
+    // submix: 5.1 (the types are Stereo, 5.1, Adaptive, Mono)
+    d.click("addTracks.submix.type");
+    assert!(
+        d.ids("addTracks.submix.type.option.").iter().any(|i| i.ends_with(".stereo"))
+            && !d.ids("addTracks.submix.type.option.").iter().any(|i| i.ends_with(".standard"))
+    );
+    d.click("addTracks.submix.type.option.5.1");
+    // amounts (number fields): two video tracks, one audio, one submix
+    d.app().ui.add_tracks.video = 2;
+    d.app().ui.add_tracks.submix = 1;
+    d.frames(2);
+    assert_eq!((d.label("addTracks.video.amount"), d.label("addTracks.submix.amount")), ("2".into(), "1".into()));
+    d.click("addTracks.ok");
+    assert!(d.ids("addTracks.").is_empty(), "dialog closed");
+    let [v, a, s] = d.tracks();
+    assert_eq!(v, ["Video 1", "Video 2", "Video 3", "Video 4", "Video 5"]);
+    assert_eq!(a, ["Audio 1", "Audio 2", "Audio 3", "Audio 4"]);
+    assert_eq!(s, ["Submix 1"]);
+    let q = d.app().session.active_sequence().unwrap().clone();
+    assert!(q.video_tracks[1].items.is_empty() && q.video_tracks[2].items.is_empty(), "the new tracks are after Video 1");
+    assert_eq!(q.video_tracks[3].items[0].id, on_v2, "what was on Video 2 moved up to Video 4");
+    use filmcraft_engine::project::AudioChannels;
+    assert_eq!((q.audio_tracks[0].channels, q.audio_tracks[0].items.len()), (AudioChannels::Mono, 0));
+    assert_eq!(q.submix_tracks[0].channels, AudioChannels::Surround51);
+
+    // with a submix track in the sequence its placement can be chosen too
+    d.ok("ui.menu.invoke", json!({"id": "sequence.addTracks"}));
+    d.frames(3);
+    assert_eq!(d.label("addTracks.submix.placement"), "After Submix 1");
+    assert_eq!(
+        (d.label("addTracks.video.amount"), d.label("addTracks.audio.type")),
+        ("1".into(), "Standard".into()),
+        "the dialog starts from the defaults again"
+    );
+    // Cancel and Escape add nothing; neither does OK with every amount at 0
+    d.click("addTracks.cancel");
+    assert!(d.ids("addTracks.").is_empty());
+    d.ok("ui.menu.invoke", json!({"id": "sequence.addTracks"}));
+    d.frames(3);
+    d.ok("ui.key", json!({"key": "Escape"}));
+    d.frames(3);
+    assert!(d.ids("addTracks.").is_empty());
+    d.ok("ui.menu.invoke", json!({"id": "sequence.addTracks"}));
+    d.frames(3);
+    d.app().ui.add_tracks.video = 0;
+    d.app().ui.add_tracks.audio = 0;
+    d.frames(2);
+    d.click("addTracks.ok");
+    assert!(d.ids("addTracks.").is_empty());
+    assert_eq!(d.tracks(), [v, a, s]);
+    assert!(d.app().ui.status.is_empty(), "{}", d.app().ui.status);
+}
+
 #[test]
 fn delete_tracks_dialog_deletes_empty_tracks() {
     let mut d = Driver::demo();
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index 402319d3..6419e154 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -39,7 +39,7 @@ Methods (handlers in `crates/ui-egui/src/control.rs`):
 | `ui.playback` | `{action: play|stop|toggle, speed?}` | |
 | `ui.screenshot` | `{path?, panel?}` | PNG of the window or one panel; fails after 10 s when no frame is presented (window hidden, display asleep) |
 | `ui.resize`, `ui.focus`, `app.quit` | | `ui.focus` is the only request that activates the app and takes keyboard focus |
-| `perf.stats` | – | performance counters (also the command `perf.stats`, so `engine.execute` and MCP `command_run` reach it; headless sessions return the engine part). `decode`: GOP-cache requests / hits / `cacheHitRate`, decoder seeks, samples decoded and skipped while catching up, `draftFrames` (decoded in draft mode), `h264Threads` (frame-threading workers per H.264 decoder), `liveDecoders` (sources holding a decoder now), `cacheMB` / `cacheBudgetMB` (decoded frames all sources hold, and the shared budget), `planePoolMB` / `planesReused` (idle plane buffers of evicted frames, and planes decoded into a recycled one), decoder ms (total and per sample), `framesDecoded`, `hardware` (Settings ▸ Playback ▸ Hardware decoding: `enabled`, pictures from hardware decoders `frames` vs `softwareFrames`, `sessions`, `declined` streams, mid-stream `fallbacks`); `playback`: playing, shown / dropped frames and `dropRate` of the current or last play, resolution, `draftDecode` (Settings ▸ Playback ▸ Draft decoding); `frames`: frame-worker jobs, cancelled, `requestHitRate`, `decodeMs` / `renderMs` (mean, p50, p95 of the last 256 jobs), queue, render-cost estimate, cache use; `ui`: fps and frame ms; `process`: CPU seconds; `media`, `jobs`. Counters are cumulative: diff two readings to measure an interval ([performance.md](performance.md)) |
+| `perf.stats` | – | performance counters (also the command `perf.stats`, so `engine.execute` and MCP `command_run` reach it; headless sessions return the engine part). `decode`: GOP-cache requests / hits / `cacheHitRate`, decoder seeks, samples decoded and skipped while catching up, `draftFrames` (decoded in draft mode), `h264Threads` (frame-threading workers per H.264 decoder), `liveDecoders` (sources holding a decoder now), `cacheMB` / `cacheBudgetMB` (decoded frames all sources hold, and the shared budget), `planePoolMB` / `planesReused` (idle plane buffers of evicted frames, and planes decoded into a recycled one), decoder ms (total and per sample), `framesDecoded`, `hardware` (Settings ▸ Playback ▸ Hardware decoding: `enabled`, pictures from hardware decoders `frames` vs `softwareFrames`, `sessions`, `declined` streams, mid-stream `fallbacks`); `playback`: playing, shown / dropped frames and `dropRate` of the current or last play, resolution, `draftDecode` (Settings ▸ Playback ▸ Draft decoding), `audio` (desktop: sound is mixed 200 ms ahead of the device; device `callbacks`, `underruns` and `missingFrames` played as silence because the mixer fell behind, `minLeadMs` the least sound left buffered); `frames`: frame-worker jobs, cancelled, `requestHitRate`, `decodeMs` / `renderMs` (mean, p50, p95 of the last 256 jobs), queue, render-cost estimate, cache use; `ui`: fps and frame ms; `process`: CPU seconds; `media`, `jobs`. Counters are cumulative: diff two readings to measure an interval ([performance.md](performance.md)) |
 
 **Selection or explicit targets.** A command that acts on the selection is "not available right now" when
 nothing is selected. When `params` name the targets under a key the command documents (`clips` /
diff --git a/docs/testing.md b/docs/testing.md
index 4f52ed21..83dd97fe 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -203,7 +203,11 @@ brute-force barycentric reference, `.cube`/`.3dl` round trips), `filmcraft-gpu`
 
 Not covered headless: `ui.screenshot` (needs a real viewport), the wgpu monitor path (the harness
 runs the CPU texture path), audio output, and wall-clock playback advance (kittest frames do not
-advance real time). Use `Driver` for new UI regressions: `d.exec(command, params)`,
+advance real time). Playing sound is covered without a device: `play_ahead` tests drive the
+mixed-ahead ring with a wall-clock device (order, underruns stay in time, mixer panic, stop), and
+`cargo run --release -p filmcraft-ui-egui --example bench_audio -- <project.fcproj>` plays a real
+project's sequences through a simulated 48 kHz device and counts dropouts, mixing in the callback
+(`direct`, as before) against mixing ahead (`ahead`). Use `Driver` for new UI regressions: `d.exec(command, params)`,
 `d.ok(method, params)`, `d.frames(n)`.
 
 ### Interactive checks