After the firefight in the gas pipes area near the exit point of the map, 
there will be a gas pipe on
fire. Turn the object off in the next room, then return to the now 
extinguished pipe area..There is
a semi secret walkway there. Walk in, find the ladder, and climb up. You 
will find a health booster,
and there is a door on your left. Go into that office. Listen to the song. 
It is the main theme from
Shogo: Mobile Armor Division, an anime FPS created by Mololith. Look at the 
dry marker board.
It is the same one from an office level in Shogo: Mobile Armor Division