On the Zombies map Five, go up to the red telephones and hit the button prompted to change the background music to "Don't Back Down" by Eminem. There are three red telephones total: One by the Quick Revive machine, one on the second floor in the center of the room, and a third one in the room with the Pack-A-Punch machine.
Shoot the mannequin heads in Nuketown you'll hear "Sympathy for the Devil" by the Rolling Stones.
On the Zombie map "Kino Der Toten" there are three pieces of a meteorite located throughout the map. The first is near the stairs when you start the level, the second is in the "boiler room", and 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 three pieces are found, "115" by Elena Siegman will play.