Because you always start on the same moment in the game, (0 days, 0 hours.. the beginning..) it always starts with same music (programmed in GTA:VC).... If you load up a game (single player), get a car, listen to radio, you ALWAYS get the same... And if you load another game (not the same) than it gives other music.. so when the music starts, depends on what time you are in the game