To unlock Squeeky Voice, pause the game and hold Left Trigger. Then press Y, X, Y, Y.

To unlock Welsh Mode, pause the game and hold the Left Trigger. Then press Y, Y, Y, Y.

To unlock Mutant Mode (which is really Big Head Mode), pause the game and hold the Left Trigger. Then press Y, B, X, X.

To unlock First Person Mode, pause the game and hold the Left Trigger. Then press Y, B, Y, B.

This is for the mini-games only. Pause while playing, hold L and press Y, B, X, X to toggle Turbo Mode.

Pause during the game, hold L and press Y, Y, Y, B to toggle the cheat.

Enable the first person mode cheat. After you go through a game of Roller Ball with that cheat in effect, your player will be invisibile (but still there).