Of bigger security concern than hacker cracks for music, is the fact that for it to be useful on a computer it has to be output to speakers. Once it makes the jump to sound waves, it is susceptible to being copied in the free and clear. In fact, it is quite trivial for any computer user to capture any melodies played on their computer. Then any content which comes out the computer's speaker, including streamed music which never resides on the hard disk, can be captured in unprotected digital form. This process is how many are using their PCs to time shift radio shows for later listening.
Encryption is one tool often mentioned when talking about securing music.
It is a way to scramble data to make it unreadable if it is intercepted. But once it is received, it has to be unscrambled to be used and loses its security blanket. An example would be a phone call. I can use encryption to scramble a phone call between me and my bookie so that anyone trying to listen in would only hear static. However, either of the listeners could have a tape deck recording the actual phone conversation in unencrypted form.