I have an Xbox, which direct plays everything as it should. However, I don’t want to have to turn on another device, nor use a controller just to Plex.
I had managed to find that thread myself in my searching, and yes, if I incorrectly set the passthrough to optical, it forces transcoding, but it’s degraded transcoding: AC3 5.1 vs EAC3 7.1. So that’s not a solution.
I’ve done more searching, and it seems like I’m not alone in experiencing these issues, which seem to stem back at least a couple of years. Unfortunately, there seems to have been little to no official response or support from Plex regarding the matter, and nothing in the way of a suitable resolution that I’ve found so far.
What I have discovered, and am now researching, are XML based client profiles, which apparently can define supported codecs etc. Unfortunately the information regarding these seems to be piecemeal and sparse at best. There is a very old and now closed thread that @PlexInfo created all the way back in 2013 Writing profiles for DLNA devices that seemed to be the early basis for what exists behind the scenes now.
But they are perhaps what is going to be the best I can do to force a solution. Hopefully I can figure it out. Or find a working example that someone has successfully used, but most of them seem to be for the Shield.