The silver ring below the black plastic secures the thermostat to the valve. If you loosen that screw, it will release pressure on the valve which in turn opens -> heating starts. It is rather tricky to fasten that ring again unless you can turn the thermostat to MAX though. Is the silver ring tight right now?
I would expect them to watch at the lowest volume level. The “muted” icon on screen would me annoy too much.