Yes, I did finally get it working, by changing i passthrough to “optical” instead of “HDMI” or “disabled”. I think the “HDMI” and “optical” options are mislabeled in the app. I tried it several times to confirm. The only way to get surround sound to the receiver over eARC or ARC is to set pass through to “optical”.
While Sony and Plex sort out this issue I have another workaround that might interest some users.
Enable DLNA support in Plex and use the inbuilt Sony Media Player.
When this issue existed for Android TV version 7 this was my workaround.
I think the DTS issue was fixed in Android TV version 8.
But I think it might have been introduced. I started to notice it again after I upgraded to Android TV version 9.
So if my media is DTS I will switch over to the Default Media Player app and connect to plex using DLNA.
I also just read the other workaround is to use Plex on my phone and Chromecast it to the TV. So I might give that a go too.
Hopefully, Sony can fix the issue soon, but I am not holding my breath.
I think there might be a better chance that Plex changes their software to work around the issue as posted above.
Is it a known bug that when setting pass-through to disabled plex outputs dts as stereo, and when setting hdmi it outputs it as AC3. seems only way to make it work is to choose optical, then both Ac3 and DTS seems to output ok. My TV is connected by optical to my receiver (old one, no src/earc support).
The only problem is when doing like this EAC3 converts to Ac3, but when choosing hdmi pass through instead it direct plays. Odd.
Another issue, when playing DTS-HD the sound is direct streamed (probably the dts core?), which is great, but at the same time 4k hdr video is transcoded. Not good at all. This must clearly be a bug in Plex client? EDIT tried another DTS-HD and then the video was direct playing and dts core direct streaming, all good, so probably something odd with the other