A Music Box is an item obtained from either Katie in Animal Crossing: Wild World and Animal Crossing: City Folk or Cyrus in Animal Crossing: New Leaf. It is a square box that will play the main theme of Animal Crossing when opened.
In Wild World and City Folk
In Wild World and City Folk, the Music Box is obtained from Katie after returning her to her mother, Kaitlin, in another town. After escorting her back to her mom, she will mail the player a gift, containing one of nine exclusive items, one of which being a Music Box.
In New Leaf
In New Leaf, the Music Box can be obtained from Cyrus after bringing him a Giant Clam Shell, a K.K. Slider song and 3,000 Bells. After half an hour, he will return a Music Box to the player. The player can then put it in a Museum Shop exhibit or in their house. Players can hit the A button and hear the song. It does not repeat, so to hear the song again, they should hit the A button again.