According to Circular 56:
Generally, a sound recording is a recorded performance, often of another work. A sound recording must be fixed, meaning that the sounds must be captured in a medium from which they can be perceived, reproduced, or otherwise communicated.So a sound recording is what is recorded and what we listen to. According to Section 101:
“Sound recordings” are works that result from the fixation of a series of musical spoken, or other sounds, but not including the sounds accompanying a motion picture or other audiovisual work...
Then what is the medium? Turning again to Circular 56:
A phonorecord is the statutory term for a physical object that contains a sound recording, such as a digital audio file, a compact disc, or an LP. The term “phonorecord” includes any type of object that may be used to store a sound recording,including digital formats such as .mp3 and .wav files.As Section 101 says:
“Phonorecords” are material objects in which sounds, other than those accompanying a motion picture or other audiovisual work, are fixed by any method now known or later developed, and from which the sounds can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device.Since 1790, when the U.S. enacted its first copyright law, the law has played catch-up with technology. Therefore, I like that it now try to include technologies that have not yet been developed ("any method now known or later developed"). So far, that forward looking phrase seems to be working for us!