From Nookipedia, the Animal Crossing wiki
K.K. Island is a song by K.K. Slider, introduced in Animal Crossing: New Leaf. When asked to play this song, K.K. will refer to it as a "secret song." It was previously used on Animal Island. Blaire, Kevin, Lyman, Medli, O'Hare, Plucky and Tangy play it in their house if they have a stereo.