All Zombie Songs

This side quest has players interact with four buttons in the Nacht Der Untoten area of the map within a short time between each other, unlocking a radio that can be shot to start playing the past map songs.

You can change the current song playing on the radio by shooting it again.

Four Button Locations

In co-op, you will have less time between each button to press the next one so having your teammate help out may be required for you to complete it.