If you know any of Aaron Copland's music, then you probably know "Hoedown," the finale of "Rodeo," the score that Mr. Copland wrote in 1942 for Agnes de Mille's ever-popular ballet about love among the cowpokes. "Hoedown" is a high-stepping orchestral fantasy based on "Bonaparte's Retreat," a 19th-century fiddle tune that Mr. Copland ran across in "Our Singing Country," a 1941 book co-edited by Alan Lomax, the celebrated folk-song collector. The version of "Bonaparte's Retreat" found in "Our Singing Country" was transcribed from a recording made by Mr. Lomax on a 1937 trip to Kentucky for the Library of Congress. It's a note-for-note rendering of the way the song was played by a fiddler named Bill Stepp. Every time you hear a symphony orchestra perform "Hoedown," you hear the ghost of Mr. Stepp's supremely virtuosic playing....