The doppler effect is AH2 is very apparently faked....sounds are lower when you are behind a plane and higher from the front.....without reguard toanything but the objects ground speed. This works great for flybys of a stationary ground target, but its pretty stupid sounding when you hear a plane you are accelerating towards sound like it is accerating away from you.....untill of course you pass the plane....at which point it sounds like it is gaining on you....as you speed away.
Basically, if you are behind it....it sounds like its moving away
if you are in front of it...it sounds like it is moving towards you
No relation to the relative motion appears to be effecting it.