Amplitude

Get Fuzzy Mode:
Press R3, R3, R3, R3, L3, L3, L3, L3, R3 during gameplay

Make notes turn into Monkey Heads:
Press L3, L3, L3, L3, R3, R3, R3, R3, L3 during gameplay

Get Tunnel View:
Press L3, L3, L3, R3, R3, R3, L3, R3, L3 during gameplay

Extra Parts:
Finish the bonus song in each level to get the extra parts.

Extra Songs:
Get the high score for the main quest menu to unlock a extra song for each area