I don't know the details, and andyhunter probably has a better idea, but I know that sonar can be used in active (emit and detect sound) or passive (detect sound from other objects).
And you would be correct. Furthermore to use active sonar is to give away your position so we rarely do.

The maps in use did not have the mountain listed so the route they layed out to travel was unfortunately through it. And a sounding was taken a couple of minutes before and returned satisfactory depth, thus the whole steeply rising terrain thing.

And yes, mountains are indeed quiet.