Course trailer from three lesson clips: one audio track in Timeline

To make a course trailer from three lesson recordings, take the best 8 to 12 seconds of each, join the audio slices with Timeline 1.0 audio.parts[] and put the matching video slots over them. audio.parts[] takes up to 20 gapless slices, each with a url and an optional source_in and duration, and joins them in the sample domain with no re-synthesis (Sume docs, read 2026-10-03). Timeline renders at $0.10 per output minute rounded up, so a 30-second trailer is one billable minute.

Pick moments first

Watch the lessons and note a start and a length for each moment. If you need the lesson audio as a separate file, run Audio detach once per lesson, since audio.parts[] slices a file by source_in. Keep the lesson files on your workspace's media.sume.com.

Write the timeline

Set duration_seconds: 30, put the three audio slices in audio.parts, and give video[] three slots whose start values are 0, 10 and 20 and whose source_in match the audio. video[0].start must be 0, and a mismatch between video and audio in-points puts a speaker's mouth out of sync, so reuse the same numbers. The error audio_parts_shorter_than_duration means your parts sum to less than the declared length. Add a short fade transition ({type:"fade",duration:0.25}) on slots two and three.

If you do not need a join, the Video trim endpoint cuts a single clip for $0.02 and is simpler.

Finish

Burn the course name and a call to action with a Video captions job and cues, $0.20. Sume does not choose the best moments for you; that is an editorial call, and a trailer made of your own lessons is a good fit for a platform that wants original content.