diff --git a/src/main.rs b/src/main.rs
index 42db7f9..9c7b2a4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,6 +16,9 @@ mod region;
 mod render;
 mod selection;
 mod shadow;
+// The sound lands in two tasks; Task 5 of the polish plan removes this.
+#[allow(dead_code)]
+mod sound;
 mod stack;
 mod thumbnail;
 mod toolbar;
diff --git a/src/sound/combo.rs b/src/sound/combo.rs
new file mode 100644
index 0000000..5a587a1
--- /dev/null
+++ b/src/sound/combo.rs
@@ -0,0 +1,71 @@
+//! Which shot of a combo this is, shared between `valw` runs.
+
+use std::path::Path;
+
+use anyhow::Result;
+
+/// The number for a shot at `now_ms`, given the previous shot.
+pub fn next(prev: Option<(u8, u64)>, now_ms: u64, reset_ms: u64) -> u8 {
+    match prev {
+        Some((n, at)) if now_ms >= at && now_ms - at < reset_ms => match n {
+            7 => 6,
+            n => n + 1,
+        },
+        _ => 1,
+    }
+}
+
+/// `(number, unix ms)` of the last shot, if the file makes sense.
+pub fn load(path: &Path) -> Option<(u8, u64)> {
+    let text = std::fs::read_to_string(path).ok()?;
+    let mut parts = text.split_whitespace();
+    let n: u8 = parts.next()?.parse().ok()?;
+    let at: u64 = parts.next()?.parse().ok()?;
+    (1..=7).contains(&n).then_some((n, at))
+}
+
+pub fn save(path: &Path, n: u8, at_ms: u64) -> Result<()> {
+    crate::output::write_atomic(path, format!("{n} {at_ms}\n").as_bytes())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn counts_up_then_alternates_six_and_seven() {
+        let mut prev = None;
+        let mut seen = Vec::new();
+        for i in 0..11u64 {
+            let n = next(prev, i * 1000, 5000);
+            seen.push(n);
+            prev = Some((n, i * 1000));
+        }
+        assert_eq!(seen, [1, 2, 3, 4, 5, 6, 7, 6, 7, 6, 7]);
+    }
+
+    #[test]
+    fn a_pause_resets() {
+        assert_eq!(next(Some((4, 1000)), 7000, 5000), 1);
+        assert_eq!(next(Some((4, 1000)), 5999, 5000), 5);
+        assert_eq!(next(None, 0, 5000), 1);
+        assert_eq!(
+            next(Some((3, 9000)), 1000, 5000),
+            1,
+            "a clock going backwards"
+        );
+    }
+
+    #[test]
+    fn state_file_round_trip_and_broken_files() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("valw-combo");
+        assert_eq!(load(&path), None);
+        save(&path, 6, 123456).unwrap();
+        assert_eq!(load(&path), Some((6, 123456)));
+        std::fs::write(&path, "garbage").unwrap();
+        assert_eq!(load(&path), None);
+        std::fs::write(&path, "9 5").unwrap();
+        assert_eq!(load(&path), None, "out of range");
+    }
+}
diff --git a/src/sound/mod.rs b/src/sound/mod.rs
new file mode 100644
index 0000000..bbbf6d2
--- /dev/null
+++ b/src/sound/mod.rs
@@ -0,0 +1,4 @@
+//! The combo shutter sound.
+
+pub mod combo;
+pub mod synth;
diff --git a/src/sound/synth.rs b/src/sound/synth.rs
new file mode 100644
index 0000000..bca7b29
--- /dev/null
+++ b/src/sound/synth.rs
@@ -0,0 +1,226 @@
+//! The shutter sounds, synthesised: no samples, no licences.
+
+use std::f32::consts::TAU;
+
+pub const RATE: u32 = 44_100;
+const SPEEDS: [f32; 5] = [1.00, 1.12, 1.25, 1.40, 1.60];
+const LOOP_SPEED: f32 = 1.80;
+const GLIDE_SECS: f32 = 0.4;
+/// The Shepard tone: octave-spaced partials from BASE Hz under a bell
+/// centred on CENTRE Hz (log scale).
+const BASE: f32 = 55.0;
+const PARTIALS: i32 = 9;
+const CENTRE: f32 = 660.0;
+const WIDTH_OCTAVES: f32 = 1.4;
+
+/// A deterministic noise source (xorshift), so every build sounds the same.
+struct Noise(u32);
+
+impl Noise {
+    fn next(&mut self) -> f32 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 17;
+        self.0 ^= self.0 << 5;
+        self.0 as f32 / u32::MAX as f32 * 2.0 - 1.0
+    }
+}
+
+fn secs(n: usize) -> f32 {
+    n as f32 / RATE as f32
+}
+
+/// A two-part mechanical "tak-shk", about 150 ms.
+pub fn shutter() -> Vec<f32> {
+    let len = (0.150 * RATE as f32) as usize;
+    let mut noise = Noise(0x1234_5678);
+    let (mut low, mut high_prev, mut high) = (0.0f32, 0.0f32, 0.0f32);
+    let mut out = Vec::with_capacity(len);
+    for i in 0..len {
+        let t = secs(i);
+        let n = noise.next();
+        // Click: bright, 0.5 ms attack, ~6 ms decay.
+        let click = n * (t / 0.0005).min(1.0) * (-t / 0.006).exp();
+        // "shk": band-passed noise from 60 ms, ~30 ms decay, softer.
+        low += 0.25 * (n - low);
+        high = 0.9 * (high + low - high_prev);
+        high_prev = low;
+        let u = t - 0.060;
+        let shk = if u >= 0.0 {
+            high * 0.9 * (u / 0.002).min(1.0) * (-u / 0.03).exp()
+        } else {
+            0.0
+        };
+        out.push(click * 0.8 + shk);
+    }
+    fade_out(&mut out, 0.010);
+    normalise(&mut out, 0.9);
+    out
+}
+
+/// `samples` played `factor` times faster (shorter and higher).
+pub fn speed(samples: &[f32], factor: f32) -> Vec<f32> {
+    let len = (samples.len() as f32 / factor).ceil() as usize;
+    (0..len)
+        .map(|i| {
+            let pos = i as f32 * factor;
+            let j = pos as usize;
+            let frac = pos - j as f32;
+            let a = samples.get(j).copied().unwrap_or(0.0);
+            let b = samples.get(j + 1).copied().unwrap_or(0.0);
+            a + (b - a) * frac
+        })
+        .collect()
+}
+
+/// The Shepard tone's partials `(frequency, amplitude)` at `position`
+/// octaves (0..1) along the glide. Amplitude depends only on frequency, so
+/// position 1 sounds like position 0.
+pub fn partials(position: f32) -> Vec<(f32, f32)> {
+    (-1..PARTIALS)
+        .map(|k| {
+            let f = BASE * 2f32.powf(k as f32 + position);
+            let d = (f / CENTRE).log2() / WIDTH_OCTAVES;
+            (f, (-0.5 * d * d).exp())
+        })
+        .collect()
+}
+
+/// A Shepard glide from `from` to `to` octaves over `secs` seconds.
+pub fn glide(from: f32, to: f32, secs_long: f32) -> Vec<f32> {
+    let len = (secs_long * RATE as f32) as usize;
+    let count = partials(0.0).len();
+    let mut phases = vec![0f32; count];
+    let mut out = Vec::with_capacity(len);
+    for i in 0..len {
+        let position = from + (to - from) * i as f32 / len as f32;
+        let mut v = 0.0;
+        for (p, (f, a)) in phases.iter_mut().zip(partials(position)) {
+            *p = (*p + TAU * f / RATE as f32) % TAU;
+            v += a * p.sin();
+        }
+        out.push(v);
+    }
+    fade_in(&mut out, 0.010);
+    fade_out(&mut out, 0.040);
+    normalise(&mut out, 0.5);
+    out
+}
+
+/// The sound for shot `n` of a combo (1–7).
+pub fn sound(n: u8) -> Vec<f32> {
+    match n {
+        1..=5 => speed(&shutter(), SPEEDS[n as usize - 1]),
+        6 | 7 => {
+            let mut s = speed(&shutter(), LOOP_SPEED);
+            let half = if n == 6 { (0.0, 0.5) } else { (0.5, 1.0) };
+            s.extend(glide(half.0, half.1, GLIDE_SECS));
+            s
+        }
+        _ => sound(1),
+    }
+}
+
+/// 16-bit mono PCM WAV.
+pub fn wav(samples: &[f32]) -> Vec<u8> {
+    let data = (samples.len() * 2) as u32;
+    let mut w = Vec::with_capacity(44 + data as usize);
+    w.extend_from_slice(b"RIFF");
+    w.extend_from_slice(&(36 + data).to_le_bytes());
+    w.extend_from_slice(b"WAVEfmt ");
+    w.extend_from_slice(&16u32.to_le_bytes());
+    w.extend_from_slice(&1u16.to_le_bytes()); // PCM
+    w.extend_from_slice(&1u16.to_le_bytes()); // mono
+    w.extend_from_slice(&RATE.to_le_bytes());
+    w.extend_from_slice(&(RATE * 2).to_le_bytes());
+    w.extend_from_slice(&2u16.to_le_bytes());
+    w.extend_from_slice(&16u16.to_le_bytes());
+    w.extend_from_slice(b"data");
+    w.extend_from_slice(&data.to_le_bytes());
+    for s in samples {
+        let v = (s.clamp(-1.0, 1.0) * 32767.0).round() as i16;
+        w.extend_from_slice(&v.to_le_bytes());
+    }
+    w
+}
+
+fn fade_in(s: &mut [f32], secs_long: f32) {
+    let n = ((secs_long * RATE as f32) as usize).min(s.len());
+    for (i, v) in s[..n].iter_mut().enumerate() {
+        *v *= i as f32 / n as f32;
+    }
+}
+
+fn fade_out(s: &mut [f32], secs_long: f32) {
+    let n = ((secs_long * RATE as f32) as usize).min(s.len());
+    let len = s.len();
+    for (i, v) in s[len - n..].iter_mut().enumerate() {
+        *v *= 1.0 - (i + 1) as f32 / n as f32;
+    }
+}
+
+fn normalise(s: &mut [f32], peak: f32) {
+    let max = s.iter().fold(0.0f32, |m, v| m.max(v.abs()));
+    if max > 0.0 {
+        for v in s.iter_mut() {
+            *v *= peak / max;
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn peak(s: &[f32]) -> f32 {
+        s.iter().fold(0.0, |m, v| m.max(v.abs()))
+    }
+
+    #[test]
+    fn the_shutter_is_short_and_never_clips() {
+        let s = shutter();
+        let ms = s.len() as f32 * 1000.0 / RATE as f32;
+        assert!((140.0..=160.0).contains(&ms), "{ms} ms");
+        assert!(peak(&s) <= 0.95 && peak(&s) > 0.3);
+        assert!(s.last().unwrap().abs() < 0.01, "ends quietly (no pop)");
+    }
+
+    #[test]
+    fn faster_is_shorter() {
+        let s = shutter();
+        assert_eq!(speed(&s, 1.6).len(), (s.len() as f32 / 1.6).ceil() as usize);
+    }
+
+    #[test]
+    fn sounds_escalate_then_loop() {
+        let lens: Vec<usize> = (1..=5).map(|n| sound(n).len()).collect();
+        assert!(lens.windows(2).all(|w| w[1] < w[0]), "{lens:?}");
+        assert!(sound(6).len() > sound(5).len(), "6 adds the glide");
+        assert_eq!(sound(6).len(), sound(7).len());
+        for n in 1..=7 {
+            assert!(peak(&sound(n)) <= 0.95, "sound {n} clips");
+        }
+    }
+
+    #[test]
+    fn the_glide_returns_to_its_start_an_octave_up() {
+        let start = partials(0.0);
+        let end = partials(1.0);
+        for (f, a) in start.iter().filter(|p| p.1 > 0.01) {
+            let twin = end
+                .iter()
+                .find(|p| (p.0 - f).abs() < 0.01)
+                .expect("same frequency an octave later");
+            assert!((twin.1 - a).abs() < 1e-3, "{f} Hz: {a} vs {}", twin.1);
+        }
+    }
+
+    #[test]
+    fn wav_header() {
+        let w = wav(&[0.0, 0.5, -0.5]);
+        assert_eq!(&w[0..4], b"RIFF");
+        assert_eq!(&w[8..16], b"WAVEfmt ");
+        assert_eq!(u32::from_le_bytes(w[24..28].try_into().unwrap()), RATE);
+        assert_eq!(w.len(), 44 + 3 * 2);
+        assert_eq!(i16::from_le_bytes(w[46..48].try_into().unwrap()), 16384);
+    }
+}