You mean: >> the cost of a pre-recorded piece of music is practically $0. The value remains the same; The sum of the marginal benefit of hearing the music each

You mean: >> the cost of a pre-recorded piece of music is practically $0. The value remains the same; The sum of the marginal benefit of hearing the music each time, which is likely greater than the $0.99 that people pay for a song on iTunes (otherwise they wouldn't pay it). If you aren't willing to pay for the song but is willing to copy it from someone's iPod if given the option then the value of the song is the opportunity cost of doing the actual copying.