It is a very small hall containing a pipe organ at the end. It consists of four pipes (two each from both sides) and a microphone.
Clicking on the keyboard causes music to play, followed by the sound of someone moaning in pain. Playing the organ to trigger the moaning sounds five times unlocks the "Johnny's Broken Heart" achievement.
Random pipe organ sounds
The organ from one of the Steam screenshots.
A view of the organ room.
A screenshot of the organ's keyboard, uploaded by Bendy from Twitter.