On Jan. 14, the violinist Hilary Hahn scored a rare gig for a classical music performer: She appeared on "The Tonight Show." And not just any "Tonight Show," but the "Tonight Show" during the final days of Conan O'Brien's brief tenure as host. Everybody was watching. So it came as no surprise that Hahn's new album, "Bach: Violin and Voice," debuted that week at No. 1 on the Billboard classical charts. No. 1 on the charts: It doesn't get any better than that. Or does it? The dirty secret of the Billboard classical charts is that album sales figures are...