KARAN ARMSTRONG, soprano, was born in Havre, Montana, and graduated from Concordia College. From a very early age, Miss Armstrong studied, practiced and performed in pursuit of a career as a concert pianist. It was not until her junior year of college (less than six years ago) that she began to sing with Thelma Halverson, protégée of London's Maggie Teyte. She was encouraged to move to Santa Barbara to study with Lotte Lehmann, as well as Tilly DeGarmo and Fritz Zweig in Los Angeles. Three key auditions resulted in three key achievements. The first, in 1964, was for the Roger Wagner Chorale; she was chosen as featured soloist for a national tour. The second audition resulted in a debut as Musetta in San Francisco Spring Opera's La Boheme in 1965. The third was for New York's Metropolitan Opera. Not only did Miss Armstrong win the auditions, but she was offered a three-year contract. During her Met seasons, the vivacious brunette has toured Europe and has appeared in New York's Town Hall and Carnegie Hall, Boston's Symphony Hall, lake George Opera Festival, Metropolitan Opera Verdi Festival at Newport, Memphis Opera and Milwaukee. last September, she made her debut with the Santa Fe Opera as Adina in Elixir of Love and this fall performed as Mimi and Susanna in lyric Opera's productions of La Boheme and The Marriage of Figaro in Laguna Beach.