Mission select:
At the Mission Select screen, click Left Analog-stick, press Black, R, click Right Analog-stick, press White, L.

Skip mission:
During a Mission, click Left Analog-stick, press White, White, Back, click Right Analog-stick, press White.

Alternate music:
At the Title screen, click Left Analog-stick twice, click Right Analog-stick twice, L, R.

Funny mission briefings:
Play missions 1, 3, 6, 8, 9 or 14. At the armory screen before launch, press L, L, R, R, click Left Analog-stick, click Right Analog-stick

Thanks to Revolution readers Kerezar, Scotty2hotty, >Project Gun Metal< and Megamer!