This exact same thing was done by a mod in WOFF, so if you find the correct file and location for WOTR, it should be simple to do for WOTR too.
I did have a nose around but couldn't figure it out. The startup sound is in the player aircraft sound folder, but AI aircraft don't have sound folders, and I guess the sounds are controlled by something that is either hidden, or beyond my understanding, or both.
Nice to hear that it was modded in WoFF though, that should mean there's no quirk of the game engine that will stop it here.