I think that for a totally realistic sound set we would need very a expensive sound synthesizing software suite that takes into account displacement of engine and a whole lot of manifold pressure and RPM ranges, along with propeller type/dimensions and external air conditions, then apply all that to mathematical functions that describe the properties of air waves and finally, match the results to what we know sounds like it in reality.
It could probably be done with today's computers and come very close to reproducing what the real thing sounded like. It would also probably be something as expensive and needing just as long a time to be coded as the rest of the simulator, so i guess we won't see it
As for noise levels, it depends on a lot of things. In the B-25 video you can see the hair of the guy in the back moving in the wind, so i guess the sound is louder because they are flying with the side window open. Not to say that the sound wouldn't be loud in any case, these were some huge engines and they were mostly tuned for performance, operating with unsilenced mufflers, but the effect is probably exaggerated a bit by having the window open.
Also, just like in motorbikes and cars, an air-cooled engine will have more of that "thump-thump-thump" sound (especially at lower RPM), just like the old VW beetles and air-cooled chopper bikes like the Harley models, while a liquid-cooled engine exhibits less of a "thumping growl" and more of a high pitched "whine".
Finally, the use or no use of noice cancelling headphones is another variable, but i don't know how well WW2 headphone sets were in that regard.
All in all, there's quite a lot of variables to get it right without devoting increasing amounts of time to it.