Thanks. I’m running RetroPie 4.7.1, installed with their image directly, so i don’t know details of the Raspbian exactly but I believe it’s Buster.
I like your idea of trying YouTube etc but I can’t work out how - or if it’s even possible - to start the normal Raspbian desktop with this setup? Using startx in the CLI having dropped out of emulationstation didn’t work.
The issue isn’t yet fixed but I have at least been able to reduce the volume now with alsamixer as per this other thread.