On the Zombies map Five, approach the red telephones and hit the button prompted to change the background music to "Don't Back Down" by Eminem. There are 3 red telephones in all: One by the Quick Revive machine, another on the second floor in the center of the room and a 3rd one is in the room with the Pack-A-Punch machine.
If you shoot the mannequin heads in Nuketown you'll hear "Sympathy for the Devil" by the Rolling Stones.
In the Zombie map "Kino Der Toten" there are 3 pieces of a meteorite located throughout the map. The first is by the stairs when you start the level. The second is in the "boiler room." The third is in the "dressing room." Hit the prompt when next to the piece and your character will say a line about the meteor -- when all 3 pieces have been found, "115" by Elena Siegman will play.
Also, on "Kino Der Toten," in the first room upstairs from where you start, there are 5 pictures on the right. Four of them are the characters you play as and one is blacked-out. You can examines these to hear your character say something about each one.