It has been a year since Canadian tenor Ben Heppner shocked a Toronto audience by cracking several notes and then calling a halt to his recital mid-aria. Heppner, long considered the most talented operatic tenor in a generation, has spent the last 12 months struggling to find out what it was that caused his voice to suddenly abandon him. He’s cagey about specifics, but the answer appears to have been found in the side effects from a medication he was taking. He is scheduled to return to New York’s Metropolitan Opera next month.