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.

If you want a proper audiobook server, and don't mind installing one alongside Plex, Audiobookshelf is fantastic.

Plexamp is free now. Just use that.

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.