[w]hen a user on a [P2P] network downloads a song from another user, he receives into his computer a digital sequence representing the sound recording. That sequence is magnetically encoded on a segment of his hard disk (or likewise written on other media). With the right hardware and software, the downloader can use the magnetic sequence to reproduce the sound recording. The electronic file (or, perhaps more accurately, the appropriate segment of the hard disk) is therefore a “phonorecord” within the meaning of the statute.