FYI, when you purchase digital music through iTunes/Amazon/etc, you still don't actually own anything. You are purchasing a license for personal use, which can be revoked for various reasons.
In practice I doubt this would ever be an issue, but just wanted to point out that you effectively never "own" a digital reproduction of something unless you are the actual copyright holder (or the copyright is permissive), and digital copies can be clawed back in a way that a CD or physical book you purchased cannot.
There is no difference between an mp3 file and a CD? A CD is exactly a digital copy (a book is an analog copy). What's true for one has to be true for the other.