I'm fairly certain that the USAAF aircraft had stall clickers, not buzzers. And as for the flap sounds, they're meant to be lower in the mix so as to convey a better sense of realism. Having everything up front in the mix reduces the effect of being in the cockpit, so I never have every sound at 100% volume.
Regarding Axis aircraft, the material available is either really bad, really bland, from completely different aircraft, or was recorded and cut so poorly that there's no way to make a decent sound file out of it. I do have complete sound sets for the Messerschmitts (even the 262), though I haven't quite gotten around to completing my work on them. The FW-190s are a different story, as there are authentic sounds for them, but it takes a long time to find the source material and piece it all together, especially for the Dora... So, a 190 set shouldn't be that far off.
But for the Japanese aircraft, it's not so easy. I've got authentic sounds for the Mitsubishi aircraft, though not complete sets. The Ki-61 uses the DB601, so it naturally sounds the same as a Bf-109F-4. The Ki-84, though, is a different story... The engine is similar to the Pratt & Whitney R-2800, yet it's much smaller and has about the same power output. I've got a few recordings that say they're authentic, but the information given is dubious at best, and the sound quality is bordering on pathetic. The best I can do for the Japanese aircraft that use the engine that's in the Ki-84 (don't remember the name off hand) is do more research, and find some source material that's at least close to authentic.
All of this being said, I've got complete sounds for virtually the entire planeset (save for the Typhoon and Tempest, as there's pretty much a complete lack of source material for them, oddly enough). It just takes time and effort to get everything done with the exact level of quality that is necessary, and have every sound normalised on the same basic scale I use for added depth (loud engines, loud guns, lower flaps and gears, etc.). It's a long and sometimes annoying process, but it's worth it, and you'll see that firsthand when I release my bomber pack (there's a very specific way bomber engine sounds have to be normalised so that they're displayed properly --- if the Eng file is at 100% for a four-engine bomber, the engine sounds combined will be very muddy and distorted, and it's similar for two-engine aircraft; this means that when the sound files are properly normalised, you get that distinctive "roar" that only multi-engine aircraft make).
And when I'm done with the planeset, I might venture into GV sounds.
