CheatOgre.com: Fuzion Frenzy

Get Welsh Mode
Pause gameplay, hold Left Trigger and press Y, Y, Y, Y.

Get Squeaky Voices
Pause gameplay, hold Left Trigger and press Y, X, Y, Y.

Get Mutant Mode
Pause gameplay, hold Left Trigger and press Y, B, X, X.

Get First Person Mode
Pause gameplay, hold Left Trigger and press Y, B, Y, B