criss - volume can be reduced to zero, otherwise you'd have up to 5 radios broadcasting in the cockpit.
Routinuely monitor 121.5 but that gets turned down/off if it's interferring with monitoring the primary ATC radio.
Often the 'too quiet' to 'too loud' adjustment is very, very tiny. And from 'too quiet' to 'off' is also very small. It's not uncommon to see the other guy answering and you realize your volume has shifted to the point of being 'off'.