The map is massive compared to Die Maschine and there is a ton to check out within Firebase Z.
One aspect that has been within zombies for as long as we can remember is the easter egg songs.
These are map specific and there are some steps you are going to need to take to complete this.
In order to activate the easter song for the Firebase Z map, you are going to need to find some cassette tapes scattered around the map.
First, make sure you have turned the power on within the map. Go here, to learn how to do this if you do not know how to already!
The first cassette is going to be within the upper portion of the spawn village. Right above Pack-A-Punch, nestled in a bookshelf.
The second one is going to be across the spawn area, right after you open the initial door. This tape will once again be hidden within some shelves!
The third and final tape is going to be in the actual Firebase Z portion of the map. Head right when you exit the teleporter, and the room to your immediate right is going to have the tape.
This one is the trickiest to find, as it is on the ground next to a filing cabinet.
Once you pickup this third and final tape, you will then hear the easter egg song!