Plex transcoding audio when audio track was changed

Then you cannot use 4K on your TV with the file you have.
As soon as you have both subtitles activated and you choose a different audio track than the first in the file, Plex will transcode the file.
This is a workaround because otherwise video and audio will “drift” apart and synchronicity cannot be maintained.

All you can do is to:

  • convert the subtitle into SRT format
  • remux the video file, so your desirted audio track is the first audio track.

see also Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ