In the video above Marni Nixon plays the role of Sister Sophia, from The Sound of Music. She is the first woman to sing in the clip and has two or three other solo lines as well. This is the extent of her solo performance in the movie (and for that matter, in most readily recognizable movies).
In the video below, Nixon records part of the score from The Sound of Music. This recording was made so that foreign language dubbers had a starting point to base their work off of.