These are the side Easter Eggs for Call Of The Dead, which includes a hidden soundtrack and several radios.

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Music Track
You will need to find the three meteorites around the map to play the song. When you find one, you will need to press the X button, Square button or F key (Platform dependant) when on them. The meteorites can be found at the following locations:

Location 1 >> One is around the starting area
Location 2 >> One is in the room with the PHD Flopper
Location 3 >> One is in the first room on the ship, several floors under the Power Room

The song is called "Not Ready to Die" by Avenged Sevenfold.

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Radios
Each radio will appear after you found the previous radio. The radios give a bit of back story to what happened at the space station before the zombie character came. When you find one, you will need to press the X button, Square button or F key (Platform dependant) when on them.. The radios are found at the following locations:

Location 1 >> Next to the Stamin-Up
Location 2 >> At the top of the Lighthouse
Location 3 >> Under the steel walkways near the PM63 and icy water
Location 4 >> Near the Double Tap Rootbeer
Location 5 >> On a rock in the icy water after the Speed Cola ice slide