I'm not sure which version of Animal Crossing this is for, but I'll answer the best I know. I only played the Gamecube version, but in that one K.K. Slider came to town on Saturday evenings. When he comes to town you can get one song off of him, either by requesting one you don't have by name, or you can have him play a random song you don't already own. After he gets done playing it on his guitar, you'll get a radio version you can play on your boombox back in your house.