Two years ago tenor Ben Heppner had to walk offstage in the middle of a recital in Toronto because of vocal problems. He got medical attention and stopped singing. Earlier this year he resumed singing and the problems seemed gone. Then this recital in Toronto: As the concert went on, “traces of fragility in the upper third of his voice became more obvious and oppressive. He began the second half with a frank acknowledgment of his problem. He would sing on, he said, for as long as we and he could manage. But it was never the forceful high notes that went awry. It was the sustained medium-volume singing in that upper third of the voice. The sound would waver and shred, and all the sophistication and subtlety of this fine artist would count for nothing. In the end, he sang five of the nine programmed songs, with varying degrees of distress.”