Assuming that the musician or conductor doesn't get too creative, a lock-in amplifier coupled with a FFT should be enough to do the trick. (To put it in perspec

Assuming that the musician or conductor doesn't get too creative, a lock-in amplifier coupled with a FFT should be enough to do the trick. (To put it in perspective, unless the orchestra is playing in a noisy bar, the sound of the piece should dominate from the device's perspective.)