At her concerts, Linda Ronstadt used to imagine that audience members were whispering to one another about what a terrible singer she was. She was an unusual rock star in several ways. Few others were as careful about keeping their distance from the insanity, and fewer turned away from arena adulation and the pop charts to do standards, operetta, and Mexican folk songs. Ronstadt was the most spectacular female singer of the rock era, her voice a thing of astonishing clarity and power and color. Due respect is here, in the documentary Linda Ronstadt: The Sound of My Voice. What...