In the level "Lapdog" There is a part where you have to drop into the vent and pipe area after the gates on the main floor are closed.

When down below, there will be a pipe that ignites to your right. A while later, there will be a valve to shut off the gas to this pipe. Once you have turned off the gas, head back to find a semi-secret walkway.

Proceed dow the walkway, up a ladder, and into a room. Inside will be a health booster, and a radio. Stay tuned after the radio anouncement to hear the theme song to Shogo: Mobile Armor Division