Mine Sir. It is the only vehicle when it get's up to speed that near deafens me...
Oh, I see, you want to decrease the volume of the M-8 individually.
In your Aces high folder, go to sounds, and then the folder for the M8, and then if you know how (I don't) you can edit the file called veng.
Otherwise, you can reduce the sounds for all vehicle engines from the clipboard.