There is no question that you can copy music from a source you have a right to (such as a CD you purchased) to a different source to facilitate your personal use (such as to an MP3). So I’m sure any Rav would say that is not a problem.
The issue is whether you can borrow a CD someone else owns (or an MP3 download someone else purchased), and make a copy in order to avoid paying for the CD (while returning the original to the person who paid for it), in spite of a copyright statement on the CD that says “no copying without producer’s permission, and in a situation where it is reasonable assume the copyright holder expects to get paid in part since the copyright holder is probably selling the material at this very moment(excluding something that is long since unavailable such as when you take a vinyl record form the 1930s and copy it so you can use it, and where the music is not being produced by anyone else).
However if you want to hear that it is permitted to freely copy, make sure to ask only about items you paid for or are not commercially available, since otherwise you’ll find a hard time getting a heter.