Separate audio and video tracks in a Short: the same split in Sume

Third-party roundups report that YouTube's Shorts editor now separates a Short into distinct audio and video tracks. Sume's Timeline 1.0 models the same idea: one audio spine that sets the length, plus a video array of up to 200 slots laid over it, and an optional soundtrack bed. If you want to edit the sound and the picture independently in code, that split is already how a render request is built.

YouTube's help pages (Enhance your Shorts and the audio editing page for YouTube Create, read 2026-10-03) describe volume, fade and split controls on audio layers, but neither announces separate Shorts tracks; that change comes from a third-party roundup I could not confirm. Sume's side is the Timeline 1.0 doc.

How do the two models line up?

The mapping is direct, with one difference: in Timeline the audio decides how long the program is, and video slots are placed against it.

How do I edit only the sound?

Keep the video slots and the program length, change audio and render again. Because each render is a new job with its own Idempotency-Key, you can keep the picture constant while trying three mixes. For slices of a longer track, use the audio endpoint: it concatenates or splits up to 20 parts or ranges and joins them in the sample domain, so a seam has no click from a re-encode. It writes WAV by default and bills $0.01 a job.

How do I edit only the picture?

Keep audio and swap slots. Slot start values must increase, a slot needs at least 0.2 seconds, and fit can be cover, contain, stretch or blur for clips that do not match the 1080x1920 default output. A still image in a slot is a static hold, not a camera move.

{
  "audio": {"url": "https://media.sume.com/artifacts/artf_demo/vo.wav", "duration_seconds": 20},
  "video": [
    {"source_url": "https://media.sume.com/artifacts/artf_demo/a.mp4", "start": 0, "duration": 8},
    {"source_url": "https://media.sume.com/artifacts/artf_demo/b.mp4", "start": 8, "duration": 12, "fit": "blur"}
  ],
  "soundtrack": {"url": "https://media.sume.com/artifacts/artf_demo/bed.mp3", "gain_db": -14}
}

What Sume does not do

There is no timeline UI to drag layers; every edit is a request. Sume does not apply YouTube's own effects, stickers or Audio Library music. It returns an MP4 you upload to YouTube.