Soundtrack for a 3-minute Short: one prompted track or a looped bed

A 180-second YouTube Short can take its music either from one Music Router generation or from a short bed that Timeline loops. Both cost $0.125 for the track, because Sume charges a flat $0.125 per music generation on every music model (Lyria 3.5 by default), plus $0.30 for the three Timeline render minutes: $0.425 either way.

Shorts run up to 3 minutes (YouTube Help, read 2026-10-09). The choice between the two options is a quality call, not a price call.

Two ways, same bill

Music Router rejects duration and duration_seconds, so the length goes in the prompt (for example, a 3-minute track with section timings). Timeline's soundtrack object has a loop flag and a fade_out_seconds of up to 10.

Request the track, then place it

Generate the music, import the finished file to media.sume.com, and pass it as the audio spine (a single track as spine needs no soundtrack object). If narration is the spine, use the music as the bed and set duck_db to lower it under the voice; ducking needs a real spine, not silence.

curl -X POST https://api.sume.com/v1/music-router/generate \
  -H "Authorization: Bearer $SUME_API_KEY" -H "Content-Type: application/json" \
  -H "Idempotency-Key: short-bed-001" \
  -d '{"model":"sume/music-auto","prompt":"Warm lo-fi, 84 BPM, instrumental. A 3-minute track, calm intro 0:00-0:20."}'

# in the Timeline body, as a bed under a voice spine:
#  "soundtrack":{"url":"https://media.sume.com/artifacts/artf_demo/bed.mp3","gain_db":-12,"loop":true,"fade_out_seconds":4,"duck_db":10}

Gotchas

• A prompted length is a request, not a guarantee. Check the finished duration before you plan around it.
• soundtrack.fade_out_seconds is at most 10, and edge fades together cannot exceed the output length.
• Looping a 30-second bed six times is audible; if the Short has quiet sections, prompt for a 3-minute track instead.
• If the music should be the only sound, send generate_audio false when you generate the clips.