Good question. I have seen quite a few discussions about the sound before, and oddly the raw sound files often sound great when you open them, but ingame they sound nothing like the raw ones.
And I know that there are several parameters involved into sound recording like microphone quality and such. But I have been at enough air shows and with most video's on youtube a P-51 sounds like when I heard one in real life.
Something appears to be off.
So I end up using mods myself for the WW2 aircraft and UH-1. From the modules I own, only the FA-18 sounds great out of the box. This is of course also subjective.