The materialization of music by means of notation and recording has had a number of consequences. On the one hand, it facilitates the transmission of music across space and time. On the other hand, formats have allowed music to become an excludable and rivalrous good. Formats embody a tension between music as an emergent social practice and music as means for controlling social practice. Jonathan Sterne's recent book, MP3: The Meaning of a Format, explores the subject of digital audio formats in general and the MP3 in particular. This essay problematizes Sterne's ideas of perceptual coding, compression, and format in an attempt to place them in a broader cultural and psychological context. By doing so, I hope to provide a clearer picture of who we are as musical beings and offer an alternate view of how musical formats serve us. I refine Sterne's format theory to include the idea of affordance and argue that the MP3 facilitates a new era of participatory media.