How Does the United States Copyright Office Define "Musical Composition"?

For copyright purposes, there is a difference between musical compositions and sound recordings.

A Musical Composition consists of music, including any accompanying words, and is normally registered as a work of performing arts. The author of a musical composition is generally the composer and the lyricist, if any. A musical composition may be in the form of a notated copy (for example, sheet music) or in the form of a phonorecord (for example, cassette tape, LP, or CD).

United States Copyright Office, "Copyright Registration of Musical Compositions and Sound Recordings"