For what I recall the best results were when all sounds being in mono; is then the system that make them heard at the left or the right depending on the position of the car.
If the other car is on your left, you will hear the sound on your left ear but the sound itself that the car output is a mono emission.
This is a guess, not sure about it but I think Wolfgang mentioned it once; if you have some mono and some other stereo then problems arise.
I think this is true. GPL's sound system places the sounds at specific directions and distances from the player's head. Your PC's sound system then tries to replicate that 3D positioning as best as it can. In theory, multiple speakers situated around the player's head should work better than using only two speakers in front for example.
Because of this, mono sounds should be better than stereo and mixing the two is probably not a good idea.