To a mystic, the "reality" of the external world is not more real than the reality in the mental plane, because, as this is subject to change, so all things in the mental plane are subject to change too. Two conditions must be fulfilled before external vibrations can be audible. You hear me speak because there is no wall between myself and you. A wall prevents communication. The second point is like a person speaking out in the open with the same pitch of voice as I use at this moment. You cannot hear the voice so well out in the open because the house we are in gives sound a place to echo in and be audible clearly.