MP3 convertion?

Hello,
I have a hypothetical question. Let's say that someone wants take
advantage of todays technologies. There goal is to be able to place as
much music on a single cd thereby alleviating cd storage and
maximizing efficiency. They buy a car stereo with MP3 capabilities and
 cd-recorder for there PC. They convert only there favorite legally
bought songs to MP3 format and burn them onto a CD. By doing so they
have cut there need for cd storage in half and have place different
artist on the same cd. Would this person be breaking the law? If so
how and why?