• On Linux: install/override profiles in /usr/lib/plexmediaserver/Resources/Profiles
• On Synology:

cd /volume1/@appstore/Plex Media Server/Resources/Profiles
sudo rm Chromecast.xml
sudo wget https://raw.githubusercontent.com/ambroisemaupate/plex-profiles/master/Chromecast.xml

Then restart your Plex service.

• Chromecast Ultra profile : Allow HEVC, MKV with external subtitles to be direct played. Override Chromecast.xml, if any issue with subtitle or sound revert to original profile (Chromecast.xml.bak)

• https://developers.google.com/cast/docs/media#audio-codecs
• https://forums.plex.tv/discussion/244663/chromecast-ultra-profile-doesnt-support-hevc/p2
• https://www.reddit.com/r/PleX/comments/6nodo9/plex_keeps_transcoding_hvec_to_chromacast_ultra/