[brando]

One of the first things that happened to me after I bought IL2 Sturmovik was my wife bringing back a flying helmet that she'd found on a shopping expedition somewhere. Oddly enough it is exactly the type shown on the box for that first version of IL2, with the padded bar across the crown and a hefty pair of bakelite headphones concealed in the protruding earpieces. Once strapped on it becomes difficult to hear much, and it would be even more isolating when the phones are plugged into an active R/T system.
I don't think it would prevent anyone from hearing the massive noise of an aero-engine mounted a few feet in front of the cockpit - but it would muffle and distort much of it. It would also be nothing like the sound experienced by people on the ground, with the added Doppler effects thrown in.

Back in the distant past, before motorcycle crash helmets were mandatory, it was common to take a ride bare-headed in order to listen to the subtleties of the engine noise when trying to tune it. With a helmet on it was impossible to hear nuances like chainlash or tappet rattle. Nowadays, with full-face helmets it is often more likely to be wind rush that is the major sound.

So I agree that the perceived sound from within a flying helmet is rather different from that heard with the naked ear.

B