I give up on Roku. I just got an nVidia Shield. No muss, no fuss, DTS passthrough worked from the Plex app with zero problems and proper audio sync. Yeah, it cost twice as much as my Ultra did, but at least the thing works correctly.
I just got a Roku Ultra and immediately saw audio sync issues in the Plex app. Judging by the age of this thread, it's going back in the box for a refund. Over a year and still not dealt with? Embarrassing. I thought I was upgrading from my old Roku 3.. apparently, not so much.