With performance halls shut because of the coronavirus pandemic, the best concert venue a violinist could hope for one recent October Friday was a sidewalk in the Bronx. Fiona Simon tuned her instrument as she prepared for one of her only public performances with the New York Philharmonic in months. The setting was a far cry from the orchestra's usual home at Manhattan's Lincoln Center. Traffic hummed and sirens wailed as a crew laid cables and unloaded speakers from the back of a double-parked pickup truck.