First enter this code at the title screen Circle, Circle, Circle, Circle, Triangle, Square, Square, Square, Square. After you have submitted this code go to new game in order for it to work.
At the main menu, press the following codes to hear a sound. Start a new game and you'll have extra weapons, ammo and features depending on your rank:
Private = square, circle, triangle.
Sergeant = Triangle, circle, square.
Lieutenant = R1, L2, L1, R2.
Captain = Circle, R1, circle, L1, triange, R2.
Major = L2, square, R2, Triange, L1, circle, R1.
Colonel = Triangle, square, circle, sqare, triangle, circle.
General = L1, down, L2, down, R1, down, R2, down.
Master of X-Squad = Circle, left, triangle, square, left.