Instructions
Go out onto the balcony through the door on the west wall. Pick up the small brown item on the railing.
Go back indoors and click on the music box. The music box is on the lower shelf in the center of the south wall. Tap on the brown object you picked up on the balcony, then tap on the missing section of the music box to add the brown object. This will form a keyboard on top of the music box with five keys.
Tap the appropriate keys to open the music box. If you're playing the original version, tap "1," "3," "5," "5," "2," "4" and "4." "1" is the key all the way on the left and "5" is the key all the way on the right. If you're playing version 2.0, tap "1," "3," "5," "5," "4," "3" and "2." The music box will open and reveal a blue key.