I just got done reviewing a relatively new program called MP3Guard. It in effect locks your mp3 files on the cd by encoding them to be played only by this commercial package.

Here is a quote from the website

MP3Guard MP3 Music Protection
MP3Guard: This BRAND NEW (2003/03/13) program is for musicians who want to protect MP3 files from copyright infringement. This MP3Guard Producer and MP3Guard Player allows musicians to sell MP3 files online.

For more info go here.... http://www.zappersoftware.com/mp3guard.html