Write a track contract
Before exporting an owned programme, list the required picture, audio and text tracks. For each, record its role, language, source, destination label and intended default behaviour. A container can hold multiple streams, but the presence of several streams does not tell the recipient which one is commentary or the main mix.
Keep stream identity separate from position. An input's second audio stream may become the first output audio stream after selection. A processing instruction that refers to an index must be checked against the actual output inventory, especially when optional tracks vary between source files.
Select content deliberately
FFmpeg documents explicit mapping and disposition settings. Use them to express the package requirement rather than relying on automatic stream selection. Confirm the installed version's behaviour and inspect the planned input-to-output mapping before processing the full programme. A copied video stream does not imply all accompanying text and audio were copied.
Suppose an invented source contains a main stereo mix, a director commentary and a caption track. The delivery requires the main mix and captions only. The correct acceptance question is whether those two selected assets arrived intact, not whether the new file has roughly the same size or a familiar extension.
Review labels and defaults independently
Play a known passage from every required audio track. Check the spoken language and content, then compare the labels with what you heard. A language tag can be wrong even when the track decodes. Likewise, a default flag is a preference signal rather than proof that every player will choose identically.
For subtitles, select the track and inspect character rendering and alignment with the delivered picture. If the destination cannot carry the required text format inside the chosen container, plan a supported sidecar or another documented packaging choice. Record the association so the recipient does not pair it with an older edit.
Deliver a reproducible inventory
Make a short handoff sheet showing the output streams in order, their roles, languages and any known destination limitation. Include a clear explanation of intentionally excluded commentary, attachments or alternate versions. This prevents an omission from being rediscovered as a mysterious playback fault.
Keep the clean source and the selection recipe. If the destination changes its accepted formats, you can produce a new derivative from the intended assets. A successful mux operation is only one check; completing the multitrack task requires verifying what the audience can actually select and hear or read.
Questions answered
Does marking an audio track default set its language?
No. Role, language metadata and default disposition are separate fields and need separate checks.
Can automatic export omit a track I wanted?
It can, depending on the tool and output. Verify an explicit track contract against the resulting file.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.