Just make a folder in your c:\htc\aceshigh\sound dir with the name of the plane.
If you wanted a different engine sound for the P51d ( which is eng.wav ) then you would put your new engine sound eng.wav file in the c:\htc\aceshigh\sound\p51d folder.
All .wav files in the P51d folder would override the original sounds in the sound folder.
To test, just make a P51d folder, copy the check6.wav from the sounds folder and paste it into the P51d folder and rename it eng.wav.
Now run the game and select the P51d and start its engine. The engine sounds normal when starting but will start sounding check6,check6,check6,check6,check6,check6 as you taxi down the runway. hehe
Pokie