Unlock Everything
Enter the following code at the mission select screen: Left-stick button, Black, R trigger, Right-stick button, White, L trigger.
Change Front-End Music
Left-stick button, Left-stick button, Right-stick button, Right-stick button, L trigger, R trigger.
Spoof Mission Briefing
Enter at the armory screen - only works on missions 1, 3, 6, 8, 9, and 14. L trigger, L trigger, R trigger, R trigger, Left-stick button, Right-stick button.
Complete current mission
Enter during gameplay without pausing the game: Left-stick button, white, white, back, Right-stick button, white.