No TV supports passthrough of TrueHD audio from a TV based app. This is a TV manufacturer restriction.
Therefore, Plex has to transcode the audio to a supported format such as EAC3.
If you want 4K HDR video and TrueHD audio, including TrueHD + Atmos, you will need to acquire a Nvidia Shield Pro or Xbox One, preferably connected to the Marantz receiver.
See [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k for additional details.
