Activate Secret Song in Kino Der Toten
There is a secret song for each zombie map. For Kino Der Toten, you have to find the 3 Meteor rocks. The first is on a pedestal in the main lobby behind the soda, the second one is in the dressing room near some Dummies (also on a pedestal), and the final one is in the upstairs room near the alley. The rock is on a dresser type thing next to some black marquee letters. These rocks while small are easily distinguishable because they have red veins on them. You must hold the USE button on your controller until your character says something or the veins glow red. The song is called 115 by Elena Siegman.
Nuketown Mannequin Secret
If you shoot the heads of all the mannequins in Nuketown you will hear Sympathy for the Devil by The Rolling Stones over the loud speakers.
Play the song "Don't Back Down" by Eminem in Five zombie map
There are three red phones throughout the pentagon that ring and flash. The first is located where you start after opening a set of doors in a corner. The second is in the Pack-a-Punch room. The third is in one of the rooms you see after leaving the second elevator on the floor the power is on. Just listen for a cell phone type ring and follow it to the red rotary phone. Look at them and hold X till the ringing stops and you hear a busy signal. After the third one the music starts.