What you are looking for is bottles. The first of which being in the library where you spawn,on a shelf next to a window.The second is in the infirmary, on a medical cart near the bloody bathtubs. The last is at the docks, well hidden, this one is behind a box near the plane part gate.
And there is another song Easter egg and its the one from the end of one of the trailers
Which is activated by inputting the numbers 935 in the spiral staircase tower number box thing.