TrueHD audio is not natively supported by the TV, so Plex transcodes it to a supported format. Since it is transcoded, you’re not getting anything close to lossless 192kHz/24bit audio. If available, use an AC3 version of the audio. It should direct play / direct stream.
Note that subtitles are also problematic with Plex Smart TV apps. Enabling PGS or VOBSUB subtitles forces a video transcode. If the audio is transcoding, enabling text subtities (SRT) also forces a transcode.
Nope not in Plex, Kodi or any other type of player.
It may change as various groups are working bard on implementing Dolby Vision in mkv files.
At least one Plex dev is heavily involved.