Pretty simple.
Just take your cartman sound file and rename it check6.wav, then drop it into the sounds folder in your Aces High folder. (Just in the main sounds folder ... not one of the individual plane subfolders created by mitsu's download). Doing this, btw, will overwrite the one in there... so rename/back up the old one if you want to keep it.
Now, if you want to get really creative ...
you can have a different check 6 for each plane/vehicle by placing a separate soundfile named check6.wav into each plane subfolder in your sounds folder. As long as it's an 8-bit mono .wav file it should work.
Those planes/vehicles without a check6.wav in their subfolder will draw on the default one in the main sounds folder.
e.g. I have german radio warnings in the subfolders for all of the lw planes, a british voice for the raf planes, russian in the vvs rides ... etc & etc.
Splash1