I've also encountered missing audio notes, but I just viewed it as a similar symptom to other issues that have been reported with content for a new note not being saved, or changes to an existing note being lost at a later sync time.
During the beta, I remember losing audio notes 3 times in a row (within the same 15min span). I would finish the recording, click the check mark, and then as soon as the note was closed the file was deleted. No history to fall back on. Had to kill the app and restart it to get my 4th attempt to work.