When I fly in arma 3 I've noticed that the outside environment is heard too easily. Even on the ground, no engines on, and no headset on, outside sounds in a real aircraft are somewhat muffled (like being in a car). The bigger the a/c the more muffled things get. However, once the engines are on most sounds should be drowned out by the engine noise, even small arms fire shouldn't be heard, unless it was very close. Also, all aviators have headsets/helmets that allow them to chat to ATC and the aircrew via ICS, this also has noise cancellation integrated with it.

So in short pilots and crew can't hear a whole lot when in aircraft, unless its very close or very loud (like a big explosion) and even then it needs to sound very muffled and with a lot of bass.