The Doppler Effect. Basically, as you create distance between you and the orifice that is generating the sounds, more and more ghosts fill the distance. Sound does not go through ghosts very well, so the volume of the sound decreases as it travels. It's called the Doppler Effect after a man named Dr. Doppler James Effect, a mad scientist who was reluctantly drafted into the ghost war after his tomb stone didn't read "Rest in Peace".