Open the .truck file using notepad, and find these lines
soundsources
98, GraveDiggerEngine
98, GraveDiggerStarter
Once you find those, head into one of the edited files and find a sound. For example, one file may have this
soundsources
98, Zoomies
98, MonsterStarter
98, Monsterreverse
Copy that, then head back into the .truck file you were editing, and replace the GraveDiggerEngine and GraveDiggerStarter with whatever sounds you wanted .
Don't copy soundsources, they should already be in that .truck file.
Hope this helps.