Visit a website that allows you to download a particular song from the game. An example is Imeem(See Resources). Examples of songs you can get include "Audition Main Theme" and "I Want to Forget" by SoulStar.Another site you can try is Amazon.com, as they have many songs available for download as well. Imeem requires you to register, but there is no cost for that part.
Click on "download this playlist" in Imeem if you want to get all of the songs for your computer.
Download iTunes if you don't have it yet. A link should appear to do this as soon as you click on the download playlist option.
Install iTunes on your computer by opening the installation file.
Purchase the songs you want and download them onto your computer for a small fee.