Song Luke Wilson listens to on headphones and sings to girlfriend

Does anyone know the name of the song Owen Wilson is listening to on his headphones when his friend comes in the room and tells him to get off the floor. It's also the same song he sernades his girlfriend with when he comes in through her window. The only lyrics I remember are "I don't know much". I've heard the song before just don't remember who it's by. Any ideas? thanks