I left my Mac’s volume too loud last night, so today while working the hourly time announcement was bothering me. It turns out you can control the volume from the CLI. It was as easy as connecting to the Mac via SSH, then:
osascript -e "set volume output volume 10"
Where 10 is the volume you want to set. Setting it to 0, well, very similar to mutting it (which can also be done, but I didn’t bother).