Shorts audio fade in and out: map the sliders to Timeline fields

The audio tools in YouTube Create list fade in and fade out for an audio layer; I did not find the same wording on a Shorts editor page. In a Sume render there are two different fades: soundtrack.fade_out_seconds, up to 10 seconds, only for the music bed, and output.fade_in_seconds / output.fade_out_seconds, 0 to 5 seconds, which fade the whole finished program. Pick by what should go quiet: just the music, or everything including picture.

YouTube Create's tool list is from Add & edit audio, read 2026-10-03. The Sume fields are from the Timeline 1.0 doc.

Which fade do I want?

A music-only fade-out is right when speech must stay clear to the last word. An output fade is right for a finished piece that should dip to black and silence together.

Is there a fade-in for the music bed?

The documented soundtrack fields are url, gain_db, loop, fade_out_seconds and duck_db; there is no soundtrack fade-in. If you need the music to come in softly, use output.fade_in_seconds for the whole program or start the bed at a lower gain_db. I did not find a per-layer fade-in in the docs.

What is a safe example?

This body gives a 1-second fade into the picture and a 2-second music tail out on a 30-second Short.

{
  "audio": {"url": "https://media.sume.com/artifacts/artf_demo/vo.wav", "duration_seconds": 30},
  "video": [{"source_url": "https://media.sume.com/artifacts/artf_demo/clip.mp4", "start": 0, "duration": 30}],
  "soundtrack": {"url": "https://media.sume.com/artifacts/artf_demo/bed.mp3", "gain_db": -12, "fade_out_seconds": 2},
  "output": {"fade_in_seconds": 1}
}

What Sume does not do

No slider and no preview waveform. You hear the result only after the render, so start with short test programs; the plan call checks price and length but not how a fade sounds.