After the firefight in the gas pipes area close to 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 and 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 to notice that it is the same one from an office level from Shogo: Mobile Armor Division.