diff --git a/src/audio/arc.ts b/src/audio/arc.ts
new file mode 100644
index 0000000..1f73879
--- /dev/null
+++ b/src/audio/arc.ts
@@ -0,0 +1,98 @@
+import { VOICE_NAMES, type VoiceName } from './mix';
+
+// The night's shape, as gain scalars per voice.
+//
+// A club does not play the same track at 9 PM and at 1 AM, and a room that
+// sounds identical for six hours is the main reason a long shift reads as flat.
+// So the mix opens thin — kick and a bit of hat, the sound of a room hoping
+// people turn up — the bassline arrives once there is a crowd to carry it, the
+// pads land at peak, and Last Drinks strips it back so the lights coming up
+// feel earned rather than abrupt.
+//
+// Pure keyframe interpolation: no audio types here, so it is testable in node
+// and reusable by the Phase-4 DJ role, which drives this same surface by hand.
+
+/** clockMin is minutes since 21:00; the night ends at 360 (3 AM). */
+export const NIGHT_END_MIN = 360;
+
+export type ArcScales = Record<VoiceName, number>;
+
+export interface ArcKeyframe {
+  /** Minutes since 21:00. */
+  at: number;
+  label: string;
+  scales: ArcScales;
+}
+
+/**
+ * Keyframes, ascending by `at`. Deliberately sparse — the interpolation between
+ * them is what makes the build feel gradual rather than stepped.
+ */
+export const ARC: readonly ArcKeyframe[] = [
+  // 9 PM. Doors open to an empty room. Kick and a whisper of hat.
+  { at: 0, label: 'EMPTY', scales: { kick: 0.8, hat: 0.4, bass: 0, pad: 0 } },
+  // 10 PM. First real queue. The bassline turns up with them.
+  { at: 60, label: 'FILLING', scales: { kick: 0.95, hat: 0.75, bass: 0.5, pad: 0.15 } },
+  // 11 PM. Room has weight.
+  { at: 120, label: 'BUILDING', scales: { kick: 1, hat: 0.9, bass: 0.85, pad: 0.5 } },
+  // Midnight through 2 AM. Everything, all of it.
+  { at: 180, label: 'PEAK', scales: { kick: 1, hat: 1, bass: 1, pad: 1 } },
+  { at: 300, label: 'PEAK', scales: { kick: 1, hat: 1, bass: 1, pad: 1 } },
+  // 2:30. Last drinks. Pull the low end first — that is what empties a floor.
+  { at: 330, label: 'LAST DRINKS', scales: { kick: 0.9, hat: 0.7, bass: 0.55, pad: 0.85 } },
+  // 3 AM. Pads over a dying kick while the lights come up.
+  { at: NIGHT_END_MIN, label: 'LIGHTS UP', scales: { kick: 0.45, hat: 0.2, bass: 0.12, pad: 0.6 } },
+];
+
+const lerp = (a: number, b: number, t: number): number => a + (b - a) * t;
+
+/**
+ * Voice scalars at a given clock minute. Clamps outside the night rather than
+ * extrapolating — a paused or overrunning clock should hold the edge, not
+ * wander into negative gain.
+ */
+export function arcAt(clockMin: number): ArcScales {
+  const t = Number.isFinite(clockMin) ? clockMin : 0;
+
+  const first = ARC[0];
+  const last = ARC[ARC.length - 1];
+  /* istanbul ignore next -- ARC is a non-empty literal */
+  if (!first || !last) return { kick: 1, hat: 1, bass: 1, pad: 1 };
+
+  if (t <= first.at) return { ...first.scales };
+  if (t >= last.at) return { ...last.scales };
+
+  for (let i = 1; i < ARC.length; i++) {
+    const prev = ARC[i - 1];
+    const next = ARC[i];
+    if (!prev || !next || t > next.at) continue;
+    const span = next.at - prev.at;
+    const k = span <= 0 ? 1 : (t - prev.at) / span;
+    const out = {} as ArcScales;
+    for (const v of VOICE_NAMES) out[v] = lerp(prev.scales[v], next.scales[v], k);
+    return out;
+  }
+
+  /* istanbul ignore next -- the loop always returns for t inside the range */
+  return { ...last.scales };
+}
+
+/** Nearest keyframe label at or before this minute — for HUD readouts. */
+export function arcLabel(clockMin: number): string {
+  const t = Number.isFinite(clockMin) ? clockMin : 0;
+  let label = ARC[0]?.label ?? '';
+  for (const k of ARC) {
+    if (t >= k.at) label = k.label;
+  }
+  return label;
+}
+
+/**
+ * One number for "how hard is the room going", 0..1 — the mean of the voice
+ * scalars. Handy for anything that wants to follow the night's energy without
+ * caring which voice is doing it (lighting, crowd density, the DJ role's meter).
+ */
+export function arcIntensity(clockMin: number): number {
+  const s = arcAt(clockMin);
+  return VOICE_NAMES.reduce((sum, v) => sum + s[v], 0) / VOICE_NAMES.length;
+}
diff --git a/src/audio/mix.ts b/src/audio/mix.ts
new file mode 100644
index 0000000..1db45c9
--- /dev/null
+++ b/src/audio/mix.ts
@@ -0,0 +1,159 @@
+// The track's tunable surface, lifted out of TechnoEngine so it can be changed
+// while the music is playing.
+//
+// Three things need this and none of them can use a `const` in a module body:
+// the ear pass (a human turns a knob and hears it on the next beat), the night
+// arc (arc.ts scales voice levels as the night builds), and the Phase-4 DJ role,
+// whose control panel is specified as this exact surface (BUILD_PLAN §4).
+//
+// Defaults reproduce the Phase-1 engine's hardcoded values exactly, so lifting
+// them changed no sound. Anything John retunes in the ear pass gets pasted back
+// over DEFAULT_MIX via `formatMix()`.
+
+export type VoiceName = 'kick' | 'hat' | 'bass' | 'pad';
+
+export const VOICE_NAMES: readonly VoiceName[] = ['kick', 'hat', 'bass', 'pad'];
+
+export interface KickParams {
+  /** Beater pitch at the transient. */
+  startHz: number;
+  /** Where the drop lands — the body of the cone. */
+  endHz: number;
+  /** Seconds for the pitch drop. Longer reads as a bigger speaker. */
+  dropS: number;
+  attackS: number;
+  decayS: number;
+}
+
+export interface HatParams {
+  /** Highpass corner. Lower lets more of the noise body through. */
+  hpHz: number;
+  decayS: number;
+}
+
+export interface BassParams {
+  rootHz: number;
+  /** Per-note filter sweep floor and peak — the "wow" on each 16th. */
+  filterLoHz: number;
+  filterHiHz: number;
+  q: number;
+  decayS: number;
+  /** Cents of detune between the two saws. The whole character lives here. */
+  detuneCents: number;
+}
+
+export interface PadParams {
+  lpHz: number;
+  /** Length of the fade-in/out cycle, in bars. */
+  bars: number;
+}
+
+export interface TrackMix {
+  /** Post-everything output level. */
+  master: number;
+  /** Per-voice peak gains, pre-filter. */
+  levels: Record<VoiceName, number>;
+  /** Ear-pass solo/mute. A muted voice is still scheduled, just silent. */
+  enabled: Record<VoiceName, boolean>;
+  kick: KickParams;
+  hat: HatParams;
+  bass: BassParams;
+  pad: PadParams;
+  /** Filter resonance at each location — the door's peak is the "through a wall" cue. */
+  doorQ: number;
+  floorQ: number;
+}
+
+export const DEFAULT_MIX: TrackMix = {
+  master: 0.8,
+  levels: { kick: 0.9, hat: 0.25, bass: 0.11, pad: 0.05 },
+  enabled: { kick: true, hat: true, bass: true, pad: true },
+  kick: { startHz: 150, endHz: 50, dropS: 0.06, attackS: 0.008, decayS: 0.34 },
+  hat: { hpHz: 7000, decayS: 0.04 },
+  bass: { rootHz: 55, filterLoHz: 180, filterHiHz: 900, q: 8, decayS: 0.12, detuneCents: 8 },
+  pad: { lpHz: 600, bars: 8 },
+  doorQ: 6,
+  floorQ: 0.7,
+};
+
+/** Bounds every knob, so a slider (or a typo in the console) cannot blow an ear. */
+export const MIX_RANGES = {
+  master: [0, 1.2],
+  level: [0, 1.5],
+  'kick.startHz': [60, 400],
+  'kick.endHz': [25, 120],
+  'kick.dropS': [0.01, 0.3],
+  'kick.attackS': [0.001, 0.05],
+  'kick.decayS': [0.05, 1],
+  'hat.hpHz': [2000, 14000],
+  'hat.decayS': [0.005, 0.2],
+  'bass.rootHz': [30, 110],
+  'bass.filterLoHz': [60, 600],
+  'bass.filterHiHz': [300, 4000],
+  'bass.q': [0.5, 20],
+  'bass.decayS': [0.02, 0.5],
+  'bass.detuneCents': [0, 40],
+  'pad.lpHz': [200, 4000],
+  'pad.bars': [1, 16],
+  doorQ: [0.5, 20],
+  floorQ: [0.1, 4],
+} as const satisfies Record<string, readonly [number, number]>;
+
+export const clampTo = (v: number, range: readonly [number, number]): number =>
+  Number.isFinite(v) ? Math.max(range[0], Math.min(range[1], v)) : range[0];
+
+/** Deep copy — callers mutate their own mix without touching DEFAULT_MIX. */
+export function cloneMix(m: TrackMix): TrackMix {
+  return {
+    master: m.master,
+    levels: { ...m.levels },
+    enabled: { ...m.enabled },
+    kick: { ...m.kick },
+    hat: { ...m.hat },
+    bass: { ...m.bass },
+    pad: { ...m.pad },
+    doorQ: m.doorQ,
+    floorQ: m.floorQ,
+  };
+}
+
+/**
+ * Effective gain for a voice: its level, muted by `enabled`, scaled by whatever
+ * the night arc says this voice should be doing right now.
+ *
+ * Kept here rather than in the engine so the ear-pass rig can show the same
+ * number the scheduler will actually use.
+ */
+export function voiceGain(mix: TrackMix, voice: VoiceName, arcScale = 1): number {
+  if (!mix.enabled[voice]) return 0;
+  return Math.max(0, mix.levels[voice] * arcScale);
+}
+
+/** True when exactly one voice is unmuted — the ear pass's soloed state. */
+export const soloedVoice = (mix: TrackMix): VoiceName | null => {
+  const on = VOICE_NAMES.filter((v) => mix.enabled[v]);
+  return on.length === 1 ? (on[0] ?? null) : null;
+};
+
+const n = (v: number): string => (Number.isInteger(v) ? String(v) : String(Number(v.toFixed(4))));
+
+/**
+ * Emit a mix as a pasteable `TrackMix` literal.
+ *
+ * The ear pass is a human saying "more click on the kick" and me turning that
+ * into numbers; without a dump button those numbers die with the tab. This is
+ * how a tuning session becomes a diff.
+ */
+export function formatMix(m: TrackMix): string {
+  return `{
+  master: ${n(m.master)},
+  levels: { kick: ${n(m.levels.kick)}, hat: ${n(m.levels.hat)}, bass: ${n(m.levels.bass)}, pad: ${n(m.levels.pad)} },
+  enabled: { kick: true, hat: true, bass: true, pad: true },
+  kick: { startHz: ${n(m.kick.startHz)}, endHz: ${n(m.kick.endHz)}, dropS: ${n(m.kick.dropS)}, attackS: ${n(m.kick.attackS)}, decayS: ${n(m.kick.decayS)} },
+  hat: { hpHz: ${n(m.hat.hpHz)}, decayS: ${n(m.hat.decayS)} },
+  bass: { rootHz: ${n(m.bass.rootHz)}, filterLoHz: ${n(m.bass.filterLoHz)}, filterHiHz: ${n(m.bass.filterHiHz)}, q: ${n(m.bass.q)}, decayS: ${n(m.bass.decayS)}, detuneCents: ${n(m.bass.detuneCents)} },
+  pad: { lpHz: ${n(m.pad.lpHz)}, bars: ${n(m.pad.bars)} },
+  doorQ: ${n(m.doorQ)},
+  floorQ: ${n(m.floorQ)},
+}`;
+}