Powell can't hear the gunfire from the roof of Nakotomi?

I think when Powell showed up, McClane shot the two guys who ran into the office on one of the uppermost floors, but wasn't on the roof. That was the "no more table" guy and his accomplice. I'd imagine that a handgun fired inside an office building with reinforced windows, 35 or so floors up, would not be audible at street level, but that's just my guess.

Expect a load of gun nerds to show up now with stats on the models used

Nah, people overweight suffer from partial hearing loss compared to fit healthy people

They showed in one scene that the gunfire wasn't being heard on street level.