Mezzo-soprano Lorraine Hunt Lieberson is one of the sought-after stars of the classical music world, and her appearance on a concert program is a sure audience draw. But over the last year, Lieberson has become a chronic no-show, canceling engagements at the Metropolitan Opera, Carnegie Hall, and the San Francisco Opera. Officially, Lieberson’s representatives say that she is suffering from a “lower back injury,” but details have been few, and speculation is rampant that the cancer which attacked the singer five years ago may have returned.