I've never used plex so this might be completely out of the ordinary for even more normal plex use cases, but is there a web client?

I use Android, iPadOS, Fedora Linux, MacOS and possibly in the future Windows (my last job required it, my next job might too) and unless the standard plex app supports audiobooks I doubt I'll find much consistency using native apps for each of those operating systems, if they even exist on all of them