Refering to the case where a bullet is fired from a car moving at the muzzle velocity of the bullet, it will leave the barrel at the same speed it usually does, but it's velocity relative to the ground the car is driving on is two times what it would be were it at rest. However, the car will not catch up to the bullet to have it crash through the window. Gravity is acting on it and once it leaves the barrel, it will begin falling. So by the time the car catches up with it, the bullet will have fallen to the ground. (Unless there is an incredible headwind to slow the bullet down)
Once comment about
Rote 8's, first post in this thread. If I recall my physics classes correctly, the speed of sound increases with a decrease in temperature, not with an increase. That makes sense too because when it's colder, the molecules are closer together. When something disturbs them (ie sound) they don't have to move very far to crash into other molecules to propagate the sound wave. In warmer air, they're further apart so they have to travel a bit before they can pass on the wave.
I may be wrong, but that is how I believe it works. I await correction
