Read some threads from back in mid 2015 that the os doesn't support surround sound like dolby or DTS, is that still the case?
I've tried just about everything to get it to work through my optic cable to the receiver. Speakers are set to audio system, digital audio out is set to Auto 2, everything in PLEX checks out too.
Was hoping to get away from my Intel NUC when it gets to watching movies but if I can't get DTS while watching movies it's not worth the switch.
DTS surround sound definitely works for me using PLEX on the Nvidia Shield TV, so it isn't PLEX or Android TV that is the problem. It still could be the TV though, Nvidia has put in a lot of work making sure the Shield has good format support. Might just want to make sure PLEX has audio passthrough turned on.
I also have a Sony Android tv. The surround works via HDMI audio return channel. Dts is not supported, all audio gets transcoded to ac3 I believe.
Only way I can get it to work is to plug in my roku 3 to the TV and view plex through there. It then passes through the audio to my receiver.
I use MX Player Pro and just play the files over SMB via an old version of ES File Explorer and get DTS through my external audio system on my X850. I ditched Plex or other streaming servers since I can just play the files directly/locally instead of having my PC transcode video files. Works much faster too. Just don't have the pretty interface.