Could you confirm what audio format the file has that you are having these issues with? There is currently a known issue with EAC3 audio and subtitles that require transcoding playback may not start.
Hope this will be fixed. If I made few restarts (like 10 times or more), MAYBE start with subs. Even graphic is not the same as on computer (it was lower quality with Plex).
Will wait if this will be fixed before I get my Plex Pass.