1. Open up the zip and open one of the truck files.
2. Find soundsources and you should see something that looks like this
soundsources
98, Zoomies
98, MonsterStarter
98, Monsterreverse
3.Rename two of the sources one GraveDiggerEngine and the other GraveDiggerStarter so it looks like this
soundsources
98, Zoomies
98, GraveDiggerEngine
98, GraveDiggerStarter
4. After that save your changes and your truck should have sound
I hope this is useful for users who thave this problem