In the CHEATS menu, select "Enter new cheat" and hit the frets(one at a time) in a specific order to unlock something.

Code	Effect
Red, Red, Blue, Yellow, Green, Green, Green, Yellow	Air Instruments
Green, Green, Red, Red, Yellow, Red, Yellow, Blue	Always Slide
Yellow, Green, Red, Red, Green, Blue, Red, Yellow	AT&T Ball Park
Yellow, Green, Red, Blue, Blue, Blue, Blue, Red	Auto Kick
Green, Red, Yellow, Blue, Red, Yellow, Blue, Green	Extra Line 6 Tones
Green, Red, Green, Blue, Red, Red, Yellow, Blue	Flame Colors
Blue, Red, Red, Green, Red, Green, Red, Yellow	Gem Colors
Green, Blue, Red, Yellow, Yellow, Red, Green, Green	Hyper Speed
Green, Red, Yellow, Yellow, Yellow, Blue, Blue, Green	Invisible Rocker
Yellow, Yellow, Blue, Red, Blue, Green, Red, Red	Performance Mode
Red, Red, Yellow, Red, Blue, Red, Red, Blue	Star Colors
Blue, Red, Yellow, Yellow, Yellow, Yellow, Yellow, Green	Unlock Aaron Steele
Blue, Red, Blue, Blue, Yellow, Yellow, Yellow, Green	Unlock Johnny Viper
Green, Red, Blue, Green, Red, Blue, Blue, Green	Unlock Nick
Blue, Red, Green, Green, Yellow, Yellow, Yellow, Green	Unlock Rina
B,B,R,G,G,B,B,Y	Unlocks every song in the game.
Red, Green, Green, Yellow, Blue, Green, Yellow,Green	Vocal Fireball