Start any type of Nazi Zombie game, and unlock the door to the Mystery Box room. 
Go to the Mystery Box. Go over to the right, and look at a table to find a 
radio. Shoot a bullet at the radio. After a few seconds, you will hear random 
types of music such as rock, disco, opera, and some in-game music such as the 
Russian victory song, the final mission music, or the Japanese combat music. The 
music will stop in between rounds but will then start again. Every player in 
game should hear it.