A Zombies map in Call of Duty: Black Ops Cold War is not complete without its share of secrets. Treyarch makes a point to fill these areas with Easter eggs and oddities for the Zombies community to find, and the payoff always feels great. In Firebase Z, they have continued their tradition of hiding away a secret song that you can activate to play while you fight the undead. Like the Die Maschine Easter egg song, this is done by finding three cassette tapes hidden around the map. Here is how to find them.
First cassette tape
You can find the first tape as early as round two in a match. It is inside Equipment Storage (the door you unlock on your way to the teleporter to the Firebase) on a shelf filled with ammo boxes to the left of a cabinet with a keypad. Interact with it to pick it up.
Second cassette tape
The second tape will require you to travel to the Firebase through the teleporter and open the gate on the right to access the Motor Pool. On the immediate right, enter the office and look at the desk and filing cabinet. The cassette is leaning against the cabinet on the floor in between the two.
Third cassette tape
For the third tape, you need to turn on the power for the map by activating the three Aetherium generators around the Firebase. When done, go back through the teleporter to return to the village you started in. Now that power is on, you can access the rest of the building that Ravenov is in.
You can find the third and final cassette tape on a bookshelf in the Scientist Quarters. This area is up the stairs from the Armory and almost directly above the Wunderfizz machine. Interact with the cassette, and you will pick it up, and the song will soon begin playing if you or a teammate grabbed the other two as well.
From our experience, the song was kind of difficult to listen to when playing with the default audio settings. If you want to hear it better, be sure to adjust your audio settings.