When last we heard from Ben Heppner, he was walking offstage in the middle of a program complaining of vocal problems. Now he’s back, singing a program in Boston. Richard Dyer reports: “The Canadian tenor has emerged from 18 difficult months of vocal problems and cancellations. He has taken charge of his life; he must have dropped 50 pounds since the last time we saw him, and he looks terrific. And it is a relief and a joy to report that the mighty voice is back in full flood. This listener heard only one insecurely supported tone in the program, and it was quiet and in the middle register.”