Pause game and press right to enter city map screen. Simultaeously press and hold L1 and R1 then enter codes:

All music: press circle, square, circle, and square.
Fights open: press square, square, circle, circle, and square.
Ghetto City: press X, square, X, triangle, triangle, and triangle.
Puma Outfit: press triangle, X, circle, and square.
Street Races open: press triangle, triangle, X, X, and triangle.
Zombiefied: press Circle, triangle, square, X, square, and X.