All of these years, Diane Rehm’s voice: the vehicle for ordinary sentences she enunciates so emphatically that they carry their utmost weight. But creaky on the edges, hitting snags. It’s a voice reliably there at midmorning on her NPR talk show, familiar; listeners love it. Except for the inevitable detractors, who say they find Rehm grating, or schoolmarmish. Love or hate that voice, it’s illuminating to learn that a physical problem contributes to Rehm’s on-air distinctiveness.