Nine 20 s voice files, one 180 s spine: Timeline audio.parts

Put the nine files in audio.parts[] as nine { url } entries and set audio.duration_seconds to 180. Timeline joins them in the sample domain, with no gap at the seams and no re-synthesis, and the render bills ceil(180 / 60) = 3 minutes at $0.10, so $0.30. A Short can be up to three minutes (YouTube page read 2026-10-08), so 180 seconds is the top of the range; use 179 for margin if the files add up to more.

The rules for parts

audio.parts[] takes 1 to 20 slices. Each slice is a Sume-hosted url with an optional source_in and duration. You cannot send audio.url together with parts (audio_url_and_parts_exclusive), and you cannot use the top-level audio.source_in with parts (audio_source_in_requires_single_spine). If the declared part lengths add up to less than audio.duration_seconds, the request fails with audio_parts_shorter_than_duration.

Part starts and video slots

Each part begins where the last one ended, so part k starts at 20 x (k - 1) seconds on the spine: 0, 20, 40, and so on to 160. If you want a picture change on every voice line, give the video slots the same starts and a duration of 20. With nine slots, the render is below the 12-slot limit for render.strategy: "single".

With parts, the join is only inside this render. If you need the joined audio as a reusable file, for example to chapter it or to reuse it for an avatar clip, use timeline audio with operation: "concat". It returns one audio_url and segments[] with the start of each part, which you use to re-base the video slots.

• All parts must share a channel layout, or the audio job refuses with audio_parts_channel_mismatch.
• The longest spine that 20 parts can build is limited by the 1,800 s spine cap, not by the part count.
• Voice files must be on media.sume.com; import them first with POST /v1/media-imports.

Check before you pay

Run the same body on POST /v1/timeline-1.0/plan. It is unbilled and returns duration_seconds, segment_count, billable_minutes and estimated_cost_usd_micros, so for this program you should see 180 seconds and 3 billable minutes, which is 300,000 micros. A plan does not download media, so it cannot find a part that is too short or a wrong channel layout; those show up when the job runs.

When 20 parts is the limit

A 180-second script of 20-second lines needs 9 parts. A script of 6-second lines needs 30 parts for the same 180 seconds, which is more than the limit of 20. In that case, join in two steps: concat with timeline audio first (up to 20 parts into one file), and use the result and the remaining parts in the final render.