The other way that you can put music into your xBox is by transferring the music from a computer. Ensure that the computer and the Xbox are connected on the same network. Press the 'A" button on the xBox to select a source. Click on the name of the computer as the source. Press "A" button to make the selection and then press it again to play the song.
Raw, the xbox makes a great media center extender and Laura gave you an excellent answer on how to "stream" music from files accessible on your home network. Another way would be to load up a flash drive with music then plug the drive in to your xbox.