It was a time when music fans copied and traded recordings without permission. An outraged music industry pushed Congress to pass anti-piracy legislation. Yes, that time is now; it was also the era of Napster in the 1990s, of cassette tapes in the 1970s, of reel-to-reel tapes in the 1950s, even the phonograph epoch of the 1930s. Piracy, it turns out, is as old as recorded music itself.

In Democracy of Sound, Alex Sayf Cummings uncovers the little-known history of music piracy and its sweeping effects on the definition of copyright in the United States. When copyright emerged,...