first see @ Plex, 4k, transcoding, and you please read whole thread
no known smart tvs support truehd.
many smart tvs have limited subtitle support
make sure you have 5.1 audio stream selected and subtitles disabled.
if it works then you know what the problem was. (hd audio and/or subtitles).
otherwise, if you want help, more details.
are you using the fire stick or plex directly on the tv?
fire stick does not support DTS, so you must use dolby digital/ac3/5.1/stereo
https://forums.plex.tv/search?q=LG%20OLED%20C9
you don’t have plex pass, so the dashboard won’t tell you much, and you won’t get gpu transcoding.
logs. verbose OFF, debug ON.
reproduce problem > Plex Web > settings > troubleshooting > download logs > drop zip into reply here.
