Assuming you are talking about the US Copyright Office, you should check out their new website which offers on-line registration.
http://www.copyright.gov/eco/index.htmlYou might not be required to physically send in a copy. They have also combined the needed forms for music - Form SR and Form PA. At one time you would have needed to complete a form SR which would copyright the "Sound Recording" and a form PA to cover the "Performing Arts" portion - the lyrics. I think this has been combined as well into one form you can submit on-line.
I have not yet used the new on-line service so I can't give exact details; however, their website is very informative and answers every possible question you can think of under the FAQ section. My only advise is to go slowly when completing the registration and make sure you understand what you are being asked to answer.
Good luck and let me know what you think of the site.