The Kaplan Penthouse programs are part of "A Little Night Music," a series of hour-long events that originated at last year's festival. Tonight, Ax was scheduled to play sonatas by Mozart and Beethoven and music by Rameau; instead, Denk will perform Bach's Partita for Keyboard No. 6 and Schumann's Faschingsschwank aus Wien. On August 3, Denk will accompany soprano Emma Bell in songs by Mozart and Debussy.