Flying a Cozy kit plane with a small 750hp engine, I could not hear the second Cozy with a larger 1200hp engine next to me.
Aside from that, the audible noise level of an engine at full revs with wind howling across your plane at 300MPH - you won't hear anything except your own engine and thoughts.
Standing on a tarmac isn't remotely the same as sitting in a closed cockpit traveling at speed.
Heres more: The first indication pilot's had they were being shot at was the tracers flying past their cockpit, not the sound of a plane's engine 150yards behind them.
One more: Take a ride in a car thats straight pipe, no muffler, no cat, you won't hear anything outside the car.
-SW