Music-only Short: Timeline silence mode with a soundtrack bed

For a Short with music and no voice, send audio: {"mode": "silence", "duration_seconds": N} and a soundtrack with the music URL. Sume plays the bed alone: with no voice spine to mix against, the compiler skips the amix step, so the bed is not attenuated by a mix. The default gain_db is -16, so raise it if you want a louder bed. Read from the Timeline docs and compiler on 2026-10-06.

Which fields are allowed?

What if the music is shorter than the Short?

Without loop, an unlooped short bed leaves a silent tail and the compiler warns. Set soundtrack.loop to true for a seamless repeat, or trim the video to the track. A hard stop at the end is rarely what you want on a Short, so set a fade_out_seconds of one to three seconds.

{
  "audio": {"mode": "silence", "duration_seconds": 45},
  "video": [{"source_url": "https://media.sume.com/artifacts/artf_demo/reel.mp4",
             "start": 0, "duration": 45}],
  "soundtrack": {"url": "https://media.sume.com/artifacts/artf_demo/bed.mp3",
                 "gain_db": -6, "loop": true, "fade_out_seconds": 2}
}

Why not use duck_db?

Ducking lowers the bed under speech. With no speech there is nothing to duck, so duck_db in silence mode returns duck_requires_audio_spine. Drop the field.