Plex isn’t really designed for audiobooks. Check this recent reddit thread where people discussed various alternatives:
https://www.reddit.com/r/PleX/comments/w6obgq/plex_support_for_audiobooks/
Seems the most commonly recommended options are Audiobookshelf and Prologue.
Also, here’s a feature request for proper audiobook support that’s nearly 10 years old and has over 1800 votes, feel free to vote and voice your opinion there:
