The effect was even more pronounced for certain big names, especially those with crossover appeal. Violinist Janine Jansen's recording of The Four Seasons, which was promoted intensely on iTunes, achieved a remarkable 73 percent of its sales via download. Hilary Hahn's CD of Mozart sonatas got 19.7 percent of its sales through downloads; for Yo-Yo Ma's Silk Road Journeys: When Strangers Meet, the figure was 10. 7 percent.

Naxos, which began offering its entire catalog for download in October 2005, is seeing rapid growth in revenue from digital downloads, much of it from back-catalog titles that are no longer available on CD.

Of course, many barriers to classical download sales remain, including the inevitable loss of sound quality that comes with digital compression. Cellist David Finckel, who founded the ArtistLed label with pianist Wu Han, told Billboard that the label was not yet interested in digital distribution.

"[The reduction in sound quality] might not be as noticeable in pop music," he said, "but in classical recordings we're trying to create the illusion of real instruments playing in real space, and that standard just isn't available yet."