Coincidentally, I just started looking into the concept of using Plex for audiobooks and this issue of “track progress” not working came up in the process of folks answering my first couple of questions - see this thread for more discussion: Couple of noob questions about Plexamp - #7 by Panthercules
My bottom line conclusion was also that this setting does not appear to be working either (at least for my combination of PMS and Roku TVs or Plexamp on iPhone), and it looks like I’ll be going with a third party player solution for the audiobooks on my Plex server as noted in that thread.
