Need for Speed Underground vehicles:
Get gold medals in every crash event. Then, at the vehicle select screen, hold the black and white buttons.

Next song:
While playing, hold the White button.

Need for Speed Underground's traffic:
Get a Gold rank on every Crash mode level, then, at the vehicle selection screne, hold the white and black buttons while choosing your car.

Thanks to Revolution readers Bond, Button master, Shang tsung, and Wayne Brady!