Level Select
Set the Vibration level in options to 1 and both SFX and Music to 0. Then hold L1 + L2 + R1 + R2 + Circle + Square. A message will confirm you have done it right.

Ending bonuses
Successfully complete the game under the hard difficulty setting. The maniac difficulty setting and a bonus chopper and will now be unlocked. The bonus chopper flies, turns, climbs, descends, and shoots faster.