This is "normal" in this game, the sound engine used has their limitations - SoundMod, done for FPS games - in a simplified way:
Your plane move inside a "bubble of sound", so what happens behind your plane move back and you have difficult to hear, at same time sounds events that happen in front you, like hits enemy plane ~150/200 meters ahead move for your cockpit. FLAK too is barely audible.
And to make more puzzled this deficiency is more noted with high end sounds system (5.1, 7.1), with my cheap onboard sound and 3.1 speakers I a able to hear hits in my plane, but not loud like my hits in enemy plane.
In similar way - but not related with sound - the puffs of your cannons hits don't appears over enemy plane, but ~5 meters behind him, weird.
A "theory" say that this happens because the game engine was done for slow WWI planes and as the WWII planes fly more fast the things became de-synchronized.