If you’ve worked on the classical side of vocal music, either as a composer or a performer, you may have heard the old joke, “What’s the difference between art songs and pop songs?” Record sales. Air play. Pick a punchline suggesting modern commercial viability. It’s enough to reduce a musicologist to tears, hearing Our Good Man Schubert’s work compared to “Disturbia” and “Baby One More Time.” When it comes down to it, though, what were art songs back in Our Good Man Schubert’s day? …