Was thinking it would be cool if people's voices and other louder sounds (gunshots) echoed a few rooms out in the sewers beyond what normally would be expected on the surface.
I would think voices could carry over a room or two, showing general direction, with yelling carrying over many more, and gunshots ringing a ton.
Maybe even whispers could be detected a room away as something like, "You here the distinct sound of whispering voices to the ."
GATO