You are walking the path that thousands have traveled before and is littered with the bodies of dead threads everywhere about the same thing.
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playing a file locally through the built in video player is not the same as streaming it through plex.
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dts is supposed to have a lossy core for compatiblity purposes, so in theory it should work without conversion. My guess is you still have subtitles enabled.
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unless you ripped them without the other audio streams, every 4k includes multiple audio tracks for different compatiblity reasons, and typically along with different languages and behind the scenes/directors comments.
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plex client communicates with the tv which between the tv and hdmi determine what capabilities are available.
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there are a couple places you can reference to determine what is happening and why;
a) https://app.plex.tv/desktop# > status > this will show your streams and will tell you what is being direct played or transcoded and how much bandwidth is being used
b) https://app.plex.tv/desktop# > settings > console > this is the log file you can monitor some of the messages while starting a movie and it can tell you why plex has decided to transcode. (try filter “MDE:” without the quotes)
c) https://app.plex.tv/desktop# > settings > troubleshooting > download logs > you can download a zip of the logs to read through in a text program of your choice.
heres another similar thread @ LG C9 eARC no DTS HD MA audio pass through
