I'm new to plex and casting but wanted to contribute something here. I have a Denon 2015 AVR-X2200W, QNAP NAS running Plex, 2016 Vizio P65, and the Android Plex App on my phone. Casting from the Netflix app on my phone in HD and DD+ sound to the P65 (with ARC to the AVR) with 4k up scaling has been a really great experience, so I wondered if I could do the same with Plex and my own movie collection, but this is what I found:
First I created some mkv files using MAKEMKV from Bluray source with DTS HD MA, Dolbty True HD, and DVD source with Dolby Digital and got them into the plex server.
1) As noted above, when casing the bluray source MKVs, I don't get lossless audio, they get converted to Dolby Digital. Not the end of the world as I only have a 5.1 set up, but I would love to see this improved to get the native lossless sound or DD+. Note: I can get the DTS HD MA native audio to play from the same MKV file using a simple DNLA stream via my Sony Bluray Player as a workaround.
On the Plex android app, in video settings I have all the boxes checked that player is AC3, EAC3 and DTS capable. I have played with advanced player settings of stream or direct play to no effect.
As a newby I'm not sure what I can do with the script above and if anything can be modified on my QNAP NAS version of plex anyway.
2) I wonder if I found a AC3 DD casting bug when there are multiple 'Plex optimized' file versions:
I was having a strange problem where the DVD source MKV file would not play DD via casting from plex. The file info on plex server confirms DD AC3 5.1 but the sound when cast was only stereo. However, from another plex client on my Tivo Roamio connected to the AVR it was able to play the file with Dolby Digital 5.1 fine... wierd.
I had been using Plex to create an optimized version for mobile use (which would downgrade to smaller file size, lower bit rate and AAC stereo audio). No matter which 'play version' I selected in the app, the audio would only be stereo when cast.
In the end I realzied if deleted the optimized versions off the plex server, leaving only original MKV file, the audio was cast correctly as DD AC3 5.1.
Bug or am I doing something wrong?
Anyway hope this is helpful.