Collecting the sheet music is a bit of a treck and requires a few steps. The sheet music is stored in a locked box downstairs in the lounge. In the room where there is a record player near the kitchen. Next to the save chair is a chest. Open it up with the key and the sheet music is inside. If you do not have they key for this yet, then you will need to solve the planet puzzle. The key is inside the moon.
Make sure you have plenty of matches. Once you activate the music box a dozen ghosts appear and you might have a hard time dodging them all. Run up to the kitchen. The door that was once blocked up is now broken open and you can get out this way to go back upstairs.