I've only used Strike Fighters 1, I think the engine speed affects the play speed of the Wav. sound file. In Strike Fighters 1 this would be in the aircraft's DATA.ini. file under [Engine]. Look for: SlowPropRPM, FastPropRPM, MaxPropRPM. I have seen engine RPM speeds used, not Prop speeds. Some WW2 fighters had a gear reduction for prop speed.
XIV Spitfire
Min. Engine RPM in Fligh = 1800
Max. Engine RPM @ Takeoff = 2,750
Propeller gear reduction ratio is about 2:1 (Divide by 2).
Min Prop RPM in flight = 900
Max Porp RPM @ Takeoff = 1375
This might lower the play speed of the sound file.
I do not know if this would work in Strike Fighters 2. Back Up All Files.
Best of luck!