At the moment I'm using Audiobookshelf (completely separate from my Plex Media Server, but running on the same computer) in a Docker container, but since I don't yet have a reverse proxy set up it is LAN or downloads only. So, I would be very interested in an answer to this as well.
Edit: I just noticed your post in c/Plex; you may want to include a link to the other post in the body so that people answering can check the other to see if their answer has already been offered.
For my Jellyfin server I use Symfonium to play audiobooks. It's not FOSS, but it was well worth the $5 I paid for it due to all the audio settings available.
Just checked and it does support Plex, Emby, and Subsonic as well as Jellyfin.