'Stunned', 'shocked', and 'surprised' all emphasize that something about her voice was unexpected. They also seem a bit negative to me. I would not use any of them here without more explanation. (e.g. "He was was shocked when he heard her sing: It had been twenty years since he'd heard anyone with such a beautiful voice.")
I think you're trying to show how strong the maestro's reaction was, not that something was unexpected.