Do you know something born, the tree example was exactly the same example I was going to give but your wrong, the tree doesn't make a sound if no one is there to hear it you see, sound is vibration of the air, it needs to vibrate off our ear drum to hear it so..... No ear drums no sound
I'm sat in a room full of physicists that beg to differ chap. You won't hear it, because to do that you need an ear, but it 100% makes a sound. A compressional wave will be induced, which by deffinition is acoustic.
I don't care if your siting with Einstein, without a receiver I.e an ear drum, all that