While I have Plex, I rarely use it. Instead I have a DLNA server installed on my media server. I'm currently watching a movie (using the Roku Media Player) with a DTS Master Audio track, which my Roku 4 passes through to my AVR as just the lossy DTS core. If I watch really closely, I might see just a hair of a audio sync issue, but I have to be watching for it. And I could be imaging it because I'm looking for it. Overall, the DTS audio appears to be in sync for me. My point is that it might not be a Roku issue, but instead something with the Plex channel. If I had to guess, Plex is likely transcoding the audio, which might introduce a sync issue. Since the audio doesn't need transcoding, it's something that Plex shouldn't be doing. I do know that Plex by default will transcode videos that really don't need it, in their effort to reliably play as much content as possible.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.