|| ...Oh, I miss you so...|
...More than you can know...
...Tell me where, oh where, did you go...?
—Mayu singing to the tune of the box.
|Corpse Party: Book of Shadows Details
Contains a key inside.
The Music Box (小箱 Kobako, lit. "Small Box") is an item found in Corpse Party: Book of Shadows.
Corpse Party: Book of Shadows
Spoilers for Corpse Party: Book of Shadows
Episode #2; 『Demise』
Yoshiki and Ayumi find a box in the custodian's closet. The box itself is locked, and Ayumi gets possessed and runs off before Yoshiki could examine it closer.
Later in the chapter, Nana, Mayu, and Yoshiki find the key that matches the box and upon opening, a nostalgic tune plays briefly and Mayu sings to it.
Inside the box is another key which is later used to unlock the reference room.
Episode #5; 『Shangri-La』
Sakutaro will hear the tune from the music box during the end of the chapter.
Significant plot details end here.