"In fact, if you’re planning on filing both a copyright in a sound recording as well as the underlying musical work, you can file just Form SR. However, in order to save yourself the $30 bucks by filing both copyrights with the same form, the performers on the sound recording and the copyright claimants for the underlying musical work must be the same person or people. Otherwise, you should file a Form PA for the written song, and a Form SR for the recorded song."
from an article on copyright and publishing
http://www.starpolish.com/advice/article.asp?id=21