Overlay an image on a silent video: compose warns, keeps audio

A mute video in Sume's compose does not fail. It warns compose_video_has_no_audio and the clip still renders. Audio passes through from the video when it has any; when it has none, the Timeline 1.0 spine supplies the audio at assemble time.

This is from the Timeline compose docs, read 2026-10-01. Runway's changelog covers its editor panel; this page is about what the compose API does with sound.

What happens to the audio?

Compose takes no audio inputs; it takes the video layer's track as it is.

Is the warning something to fix?

Only if you expected sound. A silent product loop is a normal input; the warning tells you the shot has no audio of its own, and nothing else.

Where does the audio come from then?

Drop the composed MP4 into Timeline 1.0 video[]. The docs say the spine supplies the audio at assemble time, so a voiceover or soundtrack goes on the render, not on the compose. See silent video and audio mode.

How do I see the warning?

The docs do not say where the warning is surfaced, so check the job envelope. Poll it, because there is no GET /v1/timeline-1.0/compose/:id; the result carries video_url and duration_seconds.

curl https://api.sume.com/v1/jobs/$JOB_ID/result \
  -H "Authorization: Bearer $SUME_API_KEY"