Audio

Compositions handle audio in two ways: the sound of the video items themselves, and standalone audio items (music, voice-over). In both cases the audio plays in sync during preview and is mixed into an AAC track on export — you never have to touch audio buffers yourself.

Video item audio

Video items are silent by default. Set audio to also play the file's own audio track, following the same time mapping as its frames:

const composition = {
  duration: 10,
  items: [
    {
      id: 'clip',
      path: 'path/to/video.mp4',
      compositionStartTime: 0,
      startTime: 0,
      duration: 10,
      audio: { volume: 0.8 }, // or simply `audio: true`
    },
  ],
};

Audio items

An item with kind: 'audio' contributes only sound. The source can be an audio file or any video file (its audio track is used). Audio items never appear in the frames map passed to drawFrame.

{
  id: 'music',
  kind: 'audio',
  path: 'path/to/music.m4a',
  compositionStartTime: 0, // when it starts on the timeline
  startTime: 12,           // offset inside the source file
  duration: 10,
  volume: 0.3,             // 0..1, defaults to 1
}

Overlapping audio (a video's soundtrack + background music, several music items…) is mixed together.

Looping a track over the composition

Audio items don't loop by themselves. To cover a montage longer than the track, chain as many copies as needed:

for (let time = 0, i = 0; time < duration; time += track.duration, i++) {
  items.push({
    kind: 'audio',
    id: `music-${i}`,
    path: track.path,
    compositionStartTime: time,
    startTime: 0,
    duration: Math.min(track.duration, duration - time),
    volume: 0.8,
  });
}

Export settings

When the composition contains audio, the exported file gets an AAC track, configurable through the export options:

Option	Default
audioBitRate	128000	Bit rate in bits per second.
audioSampleRate	44100	Sample rate in Hz.
audioChannelCount	2	1 = mono, 2 = stereo.