In general you should simply avoid optical and use hdmi ARC, which will pass dolby digital/dts up to 7.1.
(edit: there is nothing wrong with optical/coax, and I have nothing against it, it simply an extra cable that is not needed whenever hdmi/arc is available)
note this is not HD/lossless audio, its lossy ac3/dts like from dvd.
plex will deliver what audio that the tv says it is compatible with, and will transcode the audio as needed, ie from truehd to ac3, or dts to ac3, etc.
https://www.google.com/search?&q=hdmi+arc+vs+optical
lots of info and discussion @ Plex, 4k, transcoding, and you
