@akkbar What you’re describing is possibly a different audio bug impacting Roku than what this thread is specifically about… similar but different. There are actually two different audio bugs hitting Roku users right now so you might be hitting one or even both - and the fun part is they are related!
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This thread is about an AC3\EAC3 audio delay in MKV files that’s immediate and consistent and is a Roku bug from 9.4 update, not a Plex bug. The work around for now is to turn off Direct Play in the Plex settings on Roku - you can check the post history here or in the linked threads for specifics and details. So far that workaround has been fine for most people while we wait for Roku to fix the bug detailed in this thread. If that’s hitting you, please report it to Roku and refer to Roku Media Player as impacted as well since that’s their product.
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What you’ve described may be the other bug which I’d call audio drift. If your multichannel AC3 audio is transcoding to stereo for playback it may be transcoded to MP3 now instead of AAC and that’s been causing audio drift for many people; meaning it’ll be fine for a bit and then go out of sync and FF/RW will bring it back inline for awhile before drifting further and further out of sync again. That one IS a Plex bug on the server side transcoding and impacting Roku and Chromecast. That was introduced in a recent server update. The work around for that bug is to downgrade your server version or you might be able to fiddle with your Roku audio settings if it’s transcoding to stereo unnecessarily; sometimes the “auto” setting for audio support in Roku doesn’t pick up for connected stereo equipment and you have to force multichannel audio support to avoid transcoding to stereo - that linked thread has details for both options. Of course, if it’s not transcoding to stereo anymore you may now get hit with Bug#1 of AC3 audio delay at which point you’ll need to turn off Direct Play.
Aren’t multiple audio bugs fun!?
I agree that the Roku bugs are frustrating - I looked at alternatives too but I do like Roku as a platform that usually works pretty well so I’m hoping it’ll get sorted soon.
Sorry this is so detailed but I’ve kinda felt like a “Roku audio bug guy” in these forums lately trying to get the bug addressed and help people sort out the workarounds and you presented a good opportunity to detail the two bugs out again for folks coming to these threads. 
