To hear the hidden song, find and activate the following hidden green rocks. Note: There is no prompt to interact with the rock. The first rock is located in the spawn room at the corner near the Generator Room 3 door. To find the second rock, leave the starting area and go toward Generator Room 2. Follow the route to the workshop through the trenches and locate the sign that points to No Man's Land. Unlock the bunker door, go upstairs, and search under the bunk beds that are against the wall. To find the final rock, open the door to No Man's Land and follow the signs to the Excavation Site. The rock is hidden near some crates that are in a corner of the map to the left of the Excavation Site.