From Nookipedia, the Animal Crossing wiki
Pondering is a K.K. Slider song. He might play it when the player asks for an encore (only in City Folk) and tell him their mood is lazy. It is very slow and has frequent pauses, making it feel like one is pondering, or thinking about something very carefully, often spacing out. Monty, Doc, Cobb, Claude, and Murphy have this song playing in their house if they have a stereo.