If you want to unlock every song in the game (which disables the ability to save), head to the game's title screen and insert the following code on any instrument: Red, Yellow, Blue, Red, Red, Blue, Blue, Red, Yellow, Blue.
Gold Fender Stratocaster
To unlock the Gold Fender Stratocaster, complete Solo Tour mode with the guitar.
Gold microphone
To unlock a gold microphone, score a million points or more in Solo Tour.
Silver Fender Stratocaster
To unlock the Silver Fender Stratocaster, complete the Bonus Tour option on Solo Tour with the guitar.