The "C in a circle" notice is used only on "visually perceptible
copies." Certain kinds of works--for example, musical, dramatic, and
literary works--may be fixed not in "copies" but by means of sound in an
audio recording. Since audio recordings such as audio tapes and
phonograph disks are "phonorecords" and not "copies," the "C in a
circle" notice is not used to indicate protection of the underlying
musical, dramatic, or literary work that is recorded.