Start any sort of Nazi Zombie game and unlock the door to the Mystery Box room, then g to the Mystery Box. Go over to the right and look at a table to find a radio. Shoot a bullet at the radio then wait a few seconds. You will hear random music such as rock, disco, opera, and some in game music such as the Russian victory song, the last level music, or the Japanese combat music. The music will stop in between rounds but will then start again. Every player in game can even hear it.