Enter the Cheats at the press start
screen, and if you do it right you 
will hear a noise.

BONUS CARS:
up, left, right, X, circle, Square

HIDDEN TRACK:
X, up, triangle, down, R1, L1

REVERSE CHAMPIONSHIP:
left, triangle, R1, circle, L1, down

SUPER CHAMPIONSHIP:
right, Square, left, circle, up, X