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.
To unlock the Gold Fender Stratocaster, complete Solo Tour mode with the guitar.
To unlock a golden form of any of the four instruments (guitar, bass, drums, and mic), you must beat Solo Mode on any difficulty you choose. You must also use the instrument you intend on unlocking in beating Solo Mode, so that if, for instance, you wanted to unlock the Golden Bass, you should use the bass in Solo Mode.
To unlock a Golden Microphone, score a million points or more in Solo Tour.
To unlock the Silver Fender Stratocaster, complete the Bonus Tour option on Solo Tour with the guitar.
To unlock a silver form of any of the four instruments (guitar, bass, drums, and mic), you must beat Bonus Tour Mode on any difficulty you choose. You must also use the instrument you intend on unlocking in beating Bonus Tour Mode, so that if, for instance, you wanted to unlock the Silver Bass, you should use the bass in Bonus Tour Mode.
To unlock the "transparent" series of instruments (which gives you a transparent bass, guitar, drum kit and microphone), enter the Hall of Fame on that instrument. So, if you want the transparent bass, enter the Hall of Fame as a bassist.