From Nookipedia, the Animal Crossing wiki
K.K. Bossa, called K.K. Rhythm in Doubutsu no Mori (film), is a slow, light jazzy tune that is played by K.K. Slider in all the games in the Animal Crossing series to date, based on the Brazilian music genre, bossa nova or simply "bossa". Frank appears on the album art. Goldie, Mallary, Robin, Peggy, Wendy, Cleo, Lulu, Sylvia and Margie all have K.K. Bossa playing in their homes. It is the only song to have been given lyrics - in the Animal Crossing film it was performed by K.K. Slider at the fireworks festival. Requested by Margie, it quickly becomes her theme song, representing her friendship with the main character, Ai. Despite having words, it was sung in Animalese with a translation running along the bottom of the screen.
A wave, a splash, a ripple