In Wild World and City Folk
In Wild World and City Folk, the Music Box is a square box which can be placed in the player's house and when interacted with will play the Animal Crossing theme song. The Music Box can only be obtained after the player reunites Katie with Kaitlin.
In New Leaf
In New Leaf, the Music Box is a shell with some gems on top. The player can create a Music box by starting a project with Cyrus by giving him a Giant Clam Shell and a K.K. Slider Song. The Music Box will play the song it was created with.