If he hadn't heard fire sirens, he wouldn't have heard the "loud pop" either.
The witness claimed to have heard nothing--no running footsteps, no giggling, no doorbells, no knocking on doors, nothing until the shot. Even though this allegedly had happened all up and down the block. Hmmmmmm.
My hypothesis is modern air conditioning systems demand modern thermal insulation. This, in turn, provides really tremendous sound dampening. An indoor witness probably didn't hear much at all.