Can I save progress for audio files?

[TheDreadPirate]

For your testing, how far into the audiobook were you listening? I forget where the setting is, but there is a minimum watch/listen time before playback tracking starts. Something like a minute into the content before it starts tracking progress.

And the increment it tracks is also configurable.

I have a lot of audiobooks and can assure you the expected behavior with the Books library type is that it tracks progress.