You don’t give us any information about your tv or avr or how they are connected.
Nor do you provide any settings, such as whether hdmi passthrough is enabled in both shield and plex settings.
In any case, shield should be direct playing most anything, if properly configured, it is the only mainstream device that can pass through HD audio, and handle most subtitles.
Finally, see @ Plex Media Server
plex is adding an option to disable transcoding (still a work in progress), however all that means is, if the client cannot direct play/direct stream, it WON’T PLAY AT ALL.
