At close to a mile, its hard to hear a chopper in hover. But on a clear, quite night I'd bet you could.
I'm guessing it looks to be a mile, when he zooms out hes a bit a way..
And to stay with my theory... I'd say a mile accurate with that 30mm.. hehe j/k about this last one... Don't know but those laser range finders equiped really make that gun accurate, the computers on those choppers estimate for a whole bunch of things that are associated with distance from target. wind/trajectory/movement of chopper/height/distance etc..