This is the side Easter Egg for Kino Der Toten, a hidden soundtrack.

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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 >> In the starting area near the window under the staircase on a pedestal
Location 2 >> Near the MP5K in the dressing room, behind a mannequin on a pedestal
Location 3 >> Opposite the random box in the room linking the alley to the stage. It is on shelves facing a window

The song is called "115" by Elena Siegman and Kevin Sherwood.