I reckon that along with blocking objects it blocks humans, and therefore they can't get close enough to hear anyway?

Yeah exactly, in that case anyway. But in the case of a dorm bed or something, curtains not being as thick as a door and all that, you might be able to stand across the room and still hear it, and so it would not be the effective spell to use.

And anyway, even if they didn't hear anything, if they tried to walk next to the bed and found themselves mysteriously being deflected to the center of the room... something would definitely be suspicious ;)