If I remember correctly, Julie Andrews won for Mary Poppins because she got snubbed for recreating her role in My Fair Lady movie. There were those who didn’t like the choice of Audrey Hepburn, who didn’t sing her own songs in MFL and thought that Julie Andrews got screwed. So it was said that she was awarded the Oscar as consolation for that.
According to Audrey Hepburn's biography, she learned the songs and expected to sing them in the movie, but the producers decided to use Marni Nixon's voice.