What it boils down to is you are running into the limitations of a) Plex Smart TV apps, b) LG TVs, c) running Plex on a NAS, and d) Sonos soundbars.
If you want TrueHD & TrueHD Atmos audio, you’ll need to acquire a Nvidia Shield Pro and attach it to one of the HDMI inputs on the TV. The Shield will pass TrueHD to the TV, which will then pass it to the Sonos via HDMI-eARC. You can disable dts support in the Shield (a Shield, not Plex setting). dts will then be transcoded to a supported format.
An additional advantage of using the Shield is that it direct plays PGS/VOBSUB & SRT/VTT subtitles even if the audio or video is transcoding (a Plex Android TV app feature).
Additional comments:
The Plex Xbox app can also pass TrueHD audio (announcement). I do not know how it handles Atmos audio or subtitles.
Additional info regarding using a Shield w/ Sonos: Nvidia Shield, HDMI Passthrough, Dolby Atmos, DTS and Sonos ARC
A long thread with good info regarding playing 4K video with Plex: [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
