"Book <uuid> has no readaloud filepath after processing" after changing the transcode codec mid-run

After a long overnight whisper run, processing ended with Processing for book <uuid> failed unexpectedly and Book <uuid> has no readaloud filepath after processing. Looking at the transcoded audio directory I have a mix of .mp4 and .mp3 files. I had changed the audio setting from AAC to MP3 partway through, and also switched from local to remote whisper.

Resolution: Changing the transcode codec between runs leaves the transcoded-audio directory holding files in two different formats, and the pipeline cannot assemble a readaloud from a mixed set — it aborts and the only symptom surfaced is the unhelpful "has no readaloud filepath after processing" message. Clear the transcoded audio directory for that book (the "Delete cache files" action on the book page does this) and reprocess from scratch; that resolves it. Do not pick MP3 as the transcode format: MP3 files commonly use variable bitrate, which makes accurate timestamping impossible, and users who processed with MP3 got a readaloud that drifts progressively out of sync even when the pipeline completed. Stay on AAC even when your source audio is MP3, since Storyteller re-encodes the audio either way.