Um, guys?
Aces High ALREADY HAS 3-d sounds. You just need a 3-d soundcard and more than two speakers to enjoy it.
I have a Medusa 5.1 surround sound headphone set, plugged into an Audigy 2 LS 5.1 sound card. These headphones have multiple speakers (and a subwoofer cone) in each earpiece, and can accurately reproduce noises from the front, side, back, up, and down.
I also have a TrackIR, which smoothly rotates your head around both axes. The two combined are really amazing...
Say you're facing forward, with the engine running. Now rotate your head up and down, and fully around left and right. The engine noises rotate around your head with your views.
If you think that's cool, then check this out. Use the volume sliders to greatly reduce the the sound of your engines -- say to 40%. Crack up the volume of things relating to enemy planes and enemy shots - say to 95%. Now set all the other volumes to somewhere inbetween - I like 70% or so. (I'd tell you what these sliders are, but I'm not running the game at the moment.)
Now you can actually hear other planes around you, including behind you in the blind spots! You can even hear enemy shots get fired at you and pass by!
The sounds are indeed subtle, but if you're actually listening for them, your situational awareness is vastly increased.
I've been meaning to say KUDOS to the boys for putting in 3-D sounds without mentioning it, really. So now's the time. Kudos to you!
-Llama