AI music longer or shorter than the video: prompt, loop, trim or join

When a music track and a video differ in length, pick the fix by how far apart they are. Ask for the length in the prompt, since Sume rejects a duration field. If the track is a little short, set soundtrack.loop. If it is long, the render's declared length trims it and audio.source_in sets an in-point. If you need much more, join takes with timeline audio concat, up to 20 parts.

The four tools

Each tool belongs to a different surface, so choose by where the fix should live.

Prompt first

The Music Router docs say to steer length in the prompt, and the Music docs call these creative directions rather than guaranteed settings. Treat the requested length as a target and check the artifact. Because a generation costs a fixed $0.125 whatever the length, asking for slightly more than you need and trimming is cheaper than regenerating.

Loop, trim, join

In Timeline 1.0, the render's audio.duration_seconds declares the output length. A soundtrack bed can loop to fill it and fade out for up to 10 seconds. Looping repeats the track, so write a brief that loops well: steady tempo, no big ending, and a short intro. Soft warnings for padded or looped short sources appear on the finished job and are not failures.

To make one longer file from several takes, use timeline audio concat. It joins in the sample domain, with no silence at the seams. Choose wav output, the default, if the file will be joined again or drive lip-sync; mp3 re-adds priming padding at every edge. It costs $0.01 per job.

• A fade-out beats an abrupt loop point for most ads.
• Parts must share a channel layout (audio_parts_channel_mismatch).
• Inputs must already be this workspace's media.sume.com audio; import first.

A rule of thumb

These thresholds are my own rule of thumb, not Sume limits. Under 10 percent short: loop and fade. Under half: loop with a calm brief. More than that: ask for a longer track, or join two takes with matching key and tempo so the seam is musical. Always listen to the seam. See Jobs and results for polling each job.