I can confirm that 7.5.1 on the new models (Premiere/Premiere+/Ultra) have an audio sync issue with DTS. I have file an issue with Roku a couple weeks ago, but haven't heard anything yet. It did not affect the 7.2.2 firmware, but it looks as though they have released 7.5.1 again for everyone if you manually update. The only workaround for now, and it's not a good one, is to disable DTS in the Roku audio settings. I'll update this thread when I have any more info to share.
I can confirm this isue but I have more info for you.
Audio sync yes, but happens with AC3 audio too,
There is not a problem with Plex only, it is in the Native Roku Player too.
My situation in this moment like all users with Premiere/Premiere+/Ultra, is turn off the Roku and follow your updates and use the device when the situation is fixed.
BTW, is not and audio sync problem, is a VIDEO sync problem, if you pay attention in all files if you have subtitles in srt format they keep the same sync of the audio, but the video comes faster that the audio when is loaded to play, and yes, maybe codec problems, in fact at the moment to play always the video comes cut, it never start at the moment is comming without around 2-3 seconds cut.
I can confirm that I don't have this issue in any other devices that I use to play my personal library (LG TV, AppleTV4 for example)
We still need the feature of passthought the audio in any flavor to our A/V to enjoy the movies like need to be.
The problem now is huge! Because happends not only in 4K content, happend in any size or definition, even SD files at 480p.
My bad was bought an item that.
1- For stupid company policies the APP is not available in all countries. I can download the app which is part of the devices features because I am outside of USA.
2- Audio problems all the time and lost support for 7.1 Audio support for 5.1 only in a 2017 device with 4K support, really?
3- Terrible customer service. Almost a month and the problem still there....