Warning: possibly frustrating license talk ahead! Feel free to criticize and object and clarify!
Labels on the car textures and probably the car designs as well are protected by trademark/copyright respectively. Does the lead of trigger
Sounds and backgrounds might also be affected: The LICENSE says "GPL" (does it say so specifically about the assets?) but no author/sources are named.
I suspect that sounds and models were taken 'from the web' from 'free 3d models' and 'free sounds' websites, which have terms not compatible with GPL.
As for backgrounds, I suspect they were taken from google image search result pages.
If the assets' licenses are not compatible or their source can not be identified, I would suggest that replacing them should be a target.