@m0ffx said:
'Watermarking' techniques are the only means to try and combat it, by allowing thus-copied media to be traced. It works better for video, where there's the freedom to do it, like the dot patterns hidden in film reels, unique to each cinema - if a pirate copy is made, the cinema is traced and fined/charged/whatevered by the moviemakers. Somehow however I expect privacy advocacy groups would kick up a stink if iTunes/whoever embedded personally identifying information in music downloads.
Unfortunately audio watermarking is incredibly difficult.
If you start with an uncompressed format, such as WAV, then its quite easy to alter the raw sound data to input your watermark and certainly possible to do so in such a way that the user doesn't hear a change in the output sound. MP3 is designed to strip any sound from a file that a human can't hear, so as soon as the above WAV (or alternative) is translated to a more convenient format, the watermark is naturally lost. Unless if the original watermark was so poorly constructed the user could hear it over the music.