Alignment fails with `Encountered error running SPLIT_TRACKS` and ffmpeg `Error muxing a packet`

Processing a book died partway through. The log shows an ffmpeg Error muxing a packet line immediately followed by Encountered error running SPLIT_TRACKS. The audiobook file plays fine in a normal audio player, so I assumed the file itself was OK.

Resolution: The real error is the ffmpeg one — Encountered error running SPLIT_TRACKS is just the stage that was running when ffmpeg bailed. It means ffmpeg could not read or write the audio file, and the most common cause is not a corrupt file but filesystem permissions: a file marked locked/read-only, or one Storyteller's user cannot write next to. Check the audio file's permissions and any OS-level "locked" flag, and give the Storyteller process read/write access to both the file and its directory, then re-run processing. If permissions are correct and the muxing error persists, the audio container itself is likely damaged and should be re-encoded before importing.