Mirror Mode:
Complete Story Mode with a single credit.
Infinite Ammo:
Complete Story Mode using the Automatic Weapon to get infinite ammo.
More Credits and Free Play:
Losing all your credits will add one to your starting amount, up to a maximum of nine. After which, you will earn Free Play Mode.
Wide Ammo:
Complete Story Mode using Infinite Ammo.
Crisis Mission:
Complete Story Mode.
Music Player:
Complete Crisis Mission.
Automatic Weapon:
Complete Story Mode two times.
Extra Mode in Shoot Away 2:
Beat the high score in Retro Mode.
Source? Here it is.