Plex doesn’t support audio chapters, so as far as I know there’s nothing you can do to get Plex to recognize them. The only workaround I know is to split the file into its chapters as individual tracks. More general audiobook support (including chapter selection) is the third highest voted feature suggestion (Support for audiobooks), but hasn’t been implemented.
Plex is planning some server-side support for audiobooks and other media types like ebooks, but since it’s server-side only, you’d have to use a third-party app that connects to your server to actually play the content: Fireside Follow-up: Metadata Agenda, NFO Support, and some of our future work on PMS.
