play-a-sound

name	play-a-sound
description	Play sound effects and background music, make retro sounds from nothing with the built-in synth presets, and set the volume of each bus. Use when the user wants a jump, pickup, explosion, laser, hit, or blip sound, wants music, asks about volume or muting, or asks why the game is silent.

Play a sound

The engine has sound built in: short sound effects, looping music, and a synth that makes retro beeps out of thin air. No sound files needed to start.

When to use

Use when the user wants a jump, pickup, explosion, laser, hit, or blip sound, wants background music, wants to change or mute the volume, or asks "why is my game silent".

The fastest win: a made-up sound

BT.synthPreset has six ready-made retro sounds. You do not need any files – the engine builds the sound itself: jump, pickup, explosion, laser, hit, blip.

import { AudioClip, BT } from 'blit386';

async init() {
    // Each preset is a function that describes a sound. AudioClip.synth() turns
    // that description into a playable clip. Both need await.
    this.jumpSound = await AudioClip.synth(BT.synthPreset.jump());
    this.coinSound = await AudioClip.synth(BT.synthPreset.pickup());
    return true;
}

update() {
    if (BT.isPressed(BT.BTN_A, 0)) {
        BT.soundPlay(this.jumpSound); // play it once
    }
}

Each preset takes an optional number called a seed, so you can get a different-sounding jump without writing any new code: BT.synthPreset.jump(7). The same seed always makes the same sound.

Play a sound file

Have a real .wav, .mp3, or .ogg in public/? Load it the same way:

async init() {
    this.boom = await AudioClip.load('/sounds/boom.wav');
    return true;
}

Sound effects: play, stop, check

BT.soundPlay() hands back a SoundRef – a little ticket for that one playing sound. Keep the ticket if you want to stop it or ask about it later.

this.engineHum = BT.soundPlay(this.humSound, { loop: true, volume: 0.5 });

if (BT.isSoundPlaying(this.engineHum)) {
  BT.soundStop(this.engineHum);
}

BT.soundPlay(clip, options?) options: loop, volume (0 to 1), pitch (1 is normal, 2 is an octave up), pan (-1 left, 0 middle, 1 right), priority, fadeInMs.

Music

Music is separate from sound effects: only one music track plays at a time, and it loops by default.

async init() {
    this.song = await AudioClip.load('/music/level1.mp3');
    BT.musicPlay(this.song, { fadeMs: 1000 }); // fade in over one second
    return true;
}

// later:
BT.musicStop({ fadeMs: 500 });
BT.musicVolumeSet(0.3); // quieter, so the sound effects can be heard

Volume: the three buses

Sound travels through three "buses" (think of them as three volume knobs): 'sfx' for sound effects, 'music' for music, and 'main' for everything at once.

BT.audioVolumeSet('main', 0.8); // the master knob
BT.audioVolumeSet('music', 0.4); // music only
BT.audioVolumeSet('sfx', 1); // sound effects only

The silence rule (read this before you report a bug)

Browsers refuse to make any noise until the player has clicked, tapped, or pressed a key on the page. This is a rule of the web, not a bug in your game, and every game on the internet lives with it.

What that means for you:

• BT.soundPlay() before the player interacts is thrown away silently. It does not queue up.
• BT.musicPlay() before the player interacts is remembered, and starts by itself the moment they touch anything.
• Loading (AudioClip.load) and building sounds (AudioClip.synth) work fine while everything is still silent, so init() is still the right place for them.
• BT.isAudioUnlocked tells you which side of the line you are on.

So if a title screen looks right but sounds like nothing, that is why. The usual fix is to make the first thing the player does a keypress or a click ("Press Space to start"), and everything works from then on.

render() {
    if (!BT.isAudioUnlocked) {
        BT.systemPrint(new Vector2i(80, 120), 4, 'PRESS SPACE TO START');
    }
}

Key calls

• AudioClip.load(url) (static, async) – load a sound file. AudioClip.synth(params) (static, async) – build a sound from a description.
• BT.synthPreset.jump() / .pickup() / .explosion() / .laser() / .hit() / .blip() – the six ready-made sounds. Each takes an optional seed number.
• BT.soundPlay(clip, options?) (method) – returns a SoundRef. BT.soundStop(ref, options?), BT.isSoundPlaying(ref).
• BT.soundVolumeSet(ref, value) / BT.soundPitchSet(ref, value) / BT.soundPanSet(ref, value) – change one playing sound while it plays.
• BT.musicPlay(clip, options?) / BT.musicStop(options?) / BT.musicVolumeSet(value) – methods.
• BT.audioVolumeSet(bus, value) / BT.audioVolumeGet(bus) / BT.audioMuteSet(bus, muted) / BT.isAudioMuted(bus) – methods. Buses: 'main', 'music', 'sfx'.
• BT.isAudioUnlocked / BT.isMusicPlaying – getters (no parentheses).

Notes

• Load and synth in init() with await. Forgetting await gives you a promise instead of a clip, and nothing plays.
• Play sounds from update(), where the button presses are.
• Sound works on both renderers – WebGPU and the plain Canvas 2D fallback. Unlike CRT and post-process effects, it never needs a GPU, so every player hears it.
• Audio arrived in blit386 1.3.0. If BT.soundPlay says it is not a function, your engine is older than that: run npx blit upgrade.

See docs/audio.md.