Was thinking about this the other day oddly lol. In the event of a zombie apocalypse i would say a loud car stereo system would be the last thing you would want to have unless it were to somehow disorient them. To have a stereo big enough to disorient them from a safe distance the vehicle would have to be heavvvvy, slow, bad on gas. Three things you would not want due to scarcity and maneuverability. Also it may attract them when they hear a loud noise, like "hey here I am come and eat me".
Bottom line, buy ammo instead lol.