MARGERY MACKAY the daughter of a New Hampshire minister, intended originally to become an actress but was persuaded by American composer Ernst Bacon to switch her career to music. She went to the West Coast to complete her degree at the University of Southern California where she studied opera with Dr. Carl Ebert. She was soon singing in public and appeared frequently under such auspices as the Los Angeles Philharmonic, the Guild Opera Company, and the Hollywood Bowl. Vladmir Rosing, stage director of the New York City Opera urged her to go East for auditions. She made the trip and won contracts with the New York City Opera and with Columbia Artists Management. One of her greatest thrills was opening the Lyric Opera of Chicago's season with Joan Sutherland. Another was portraying Suzuki to Antonietta's Butterfly in San Francisco. Miss MacKay is equally at home singing music of a lighter nature, recently singing the part of the Mother Abbess on the sound track of 20th Century Fox's Sound of Music.