From Nookipedia, the Animal Crossing wiki
The Main Music Theme is music that plays on the title screen.
In Animal Crossing
Animal Crossing has a different theme to Animal Crossing: Wild World and Animal Crossing: City Folk, which was changed when Animal Crossing: Wild World came into being. The game's title screen plays only a part of it, however. To listen to the whole theme, one must open the disk cover if playing on the Gamecube or remove the disk if playing on the Wii, the moment Porter the monkey appears. If opened/removed later or the player waits until another opening occurs, the opening will still continue and the song will stop as normal. This song was eventually made into the K.K. Slider song Forest Life.
The theme was changed from the original Animal Crossing GCN main theme. To replace this, Forest Life is available as a song, as of Animal Crossing: Wild World. This song is a cover in Super Smash Bros. Brawl and can be played in Wii Music. There is also an extended version of this song.