How Does the United States Copyright Office Define "Sound Recording"?

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

A Sound Recording results from the fixation of a series of musical, spoken, or other sounds. The author of a sound recording is the performer(s) whose performance is fixed, or the record producer who processes the sounds and fixes them in the final recording, or both.

Copyright in a sound recording is not the same as, or a substitute for, copyright in the underlying musical composition.

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