Level select:
While you are at the main menu press Square, Triangle, Circle, Left, Up, Right,
L2, R2.
Skip level with Silent Assasin rank and extra weapons:
When the level starts press R2, L2, Up, Down, X, L3, Circle, X, Circle, X.
Extra weapons:
Beat the following missions with a Silent Assassin rank to get the weapons.
Mission 1 - CZ 2000 Dual Pistols
Mission 2 - Micro Uzi Dual Submachine Guns
Mission 3 - Silverballer Silenced Dual Pistols
Mission 4 - Magnum 500 Dual Pistols
Mission 5 - Sawed-Off Shotgun Dual
Mission 6 - M4 Carbine Silenced Assault Rifle
Mission 7 - SG220 .S Dual Pistols
Mission 8 - MP5 Silenced Submachine Gun
Mission 9 - AK 74 Silenced Assault Rifle
Mission 10 - GK 17 Dual Pistols
Mission 11 - Micro Uzi Silenced Dual Submachine Guns
Mission 12 - PGM Silenced Sniper Rifle