That depends on the aircraft, also remember pilot has a headset on which cancels out a lot of noise. Even on the ground the other aircraft shouldn't be heard over one another. I was inside a lancaster during an engine run, once more than one engine is running you don't get an equal increase in sound.
Similar thing with the aircraft in dcs, when joining up or flying along side another aircraft I could hear the other plane just as clear as mine. Perhaps it could be muffled a bit?