My particular audio setup requires me to use the device itself to change the volume as the amplifier isn’t easily accessible. But when the Plex audio is surround sound and I try to adjust the volume (which is stuck on 100%) I got the message :-
"Surround sound enabled. To adjust volume, use your TV’s remote control”.
After trying to edit the Plex Chromecast configuration with no success, I found the solution in adjusting the Chromecast advanced settings.
In the Google Home app, select the Chromecast you want to use, and then in the top right use the cog to enter the Device settings. Scroll down to the Advanced section and change the “Surround sound mode” to “Stereo only”.
Now when playing a surround sound audio track on the Chromecast, Plex with transcode the audio down to stereo reenabling the audio slider.