Audio Subsystem

Everything the player hears, and the one rule that shapes all of it: the engine ships no audio content, so every sound has a compiled-in procedural fallback and the whole subsystem is provable with zero content packs.

The HAL is platform/IAudio.h, implemented by platform/openal/OALAudio (OpenAL Soft). Engine-side managers consume IAudio* by injection and never see an OpenAL header.

A null IAudio is a clean no-op

Every manager takes IAudio* and no-ops against a null one, which is what makes the whole subsystem testable headless — CI never opens an audio device. Nothing here is a hard dependency on a sound card: the managers hold buffers and sources they simply never hear.

Builtin PCM is byte-stable, never random

generateBuiltinSfx (engine/audio/SfxBuiltinSounds.h) and generateBuiltinMusic (engine/audio/MusicBuiltinTracks.h) synthesise their PCM from a fixed hash and IEEE float math — never rand() and never the clock. The determinism is deliberate: it makes the zero-pack sandbox sound the same on every machine and every run, which is what lets a test assert on the output at all.

Managers

Manager	Header	What it owns
SfxManager	engine/audio/SfxManager.h	Positional weapon SFX (#631): a 16-voice steal-oldest pool, a full-decode buffer cache, and camera-relative 3D positioning — play(preset, worldPos, cameraOrigin, settings) subtracts the camera origin and puts the listener at the origin, the same invariant the renderer uses. Preset vocabulary sfx.gunfire / launch / release / impact / explosion; a content-pack asset name overrides the compiled-in procedural fallback.
MusicManager	engine/audio/MusicManager.h	Streams OGG music through two OpenAL sources (primary + crossfade). loadPlaylist / setState(GameState) / update(dt, masterVol, musicVol). A shuffle state Fisher-Yates shuffles on entry and re-shuffles each loop cycle; setRng injects a deterministic RNG for tests. openSlot resolves a builtin: track before AssetManager and streams its looping PCM through the same crossfade buffers as a real OGG.
WarningToneManager	engine/audio/WarningToneManager.h	Cockpit stall + overspeed tones (#957): activation, exit hysteresis, live gain, the in-flight gate, and channel independence.
EngineAudioManager	game/fighters-legacy/EngineAudioManager.h	Continuous engine + aerodynamic layers (#959): the throttle/airspeed → pitch/gain mapping, own-ship head-locked audio, and positional flyby Doppler.
VoiceCalloutManager	engine/audio/VoiceCalloutManager.h	Crew/GCI callout playback with the subtitle queue.
VoiceMixer / VoiceChat	engine/voice/	In-game radio comms (Epic J). See Voice.

Content-pack audio

Music playlists are data/playlist.toml in a pack — see Asset formats. When no pack provides one, the client uses builtinDefaultPlaylist(), which wires builtin:music-menu / -patrol / -combat to the game states, so music plays with zero content.

Effects routing is the game layer's ClientEffectRouter, which drives SfxManager alongside the particle effects and HapticController (own-ship launch → notifyOrdnanceRelease; the missile-approach warning is deliberately withheld — it would be a wallhack, tracked on #529).

Testing

Audio is fully testable headless: no device is ever opened in CI. Every audio test drives a no-op or tracking IAudio double instead.

Sound is not verified by listening; the assertions are on buffer/source lifecycle, gain and position, state transitions, and the byte-stability of the builtin PCM. test_music_manager carries a design note worth reading before adding cases: a no-op processedBufferCount returning 0 means the EOF handler only fires when openSlot fails, so track-advance logic is driven with a pack that returns nullopt, and a pack returning valid OGG is used only for setState()-entry cases.