In the Plex app, select your username, choose Settings → About. Current Version is 4.29.6.
Are you using the TV speakers or passing audio to a receiver/soundbar?
What do you see in the audio settings for the app? Is there a choice for dts audio? If so, unselect it and see what happens. It should force Plex to transcode the audio.
According to the online specs, LG does not support dts audio on their 2020 TVs (your model anyway).
This is a change for 2020. LG TVs in prior years supported dts audio. Plex may have to update the app for the 2020 models.
