Hi. You have to goto
My Computer . Then select the
C: drive. Ok, now, you have to select
Program Files then
HTC. Select the Aces High II file and you get a bunch of other files. Go to the file named Sounds. In this file you see a list of all the planes and vehicles. Your plane is in there, but before you change the sound, go back out of the Sounds file and make another file called Sounds_2 (this is so you don't lose any of your original sounds). Now go back to the original Sounds file, then copy and paste everything into your newly created Sounds _2 file. Ok, now for the easy part:). In your Sounds_2 file scroll down the list until you see the file named 190A5. Open this file and you'll see the sounds that that are to that plane. Your looking for the 20MM guns, so just change the 20MM_3 and 20MM_4.
Phewwww... your done. Check out AHII to see if it worked.
If it didn't or you have any questions PM me.
