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.