Continuity mistake: Nicholas Hammond (Friedrich) was not a natural blond. The first scene to be shot involving him was the "My Favourite Things" scene and you can see that his hair is pale auburn. After this scene was shot, the directors thought that he would look better if his hair were lighter, so they applied an early form of bleaching agent to his hair. In scenes which were shot after this (notably the children's first entrance to meet Maria which occurs before the "Favourite Things" scene) his hair is very, VERY blond, almost white. In fact, if you scrutinise his hair closely throughout the film, it is a different colour in practically every scene, depending on when the scene was shot and how much of the bleaching agent had been applied.
Continuity mistake: During the boat scene, the baroness leaves and walks behind Maria. From the immediate opposite shot the baroness has vanished.
Answer: Postulants/Novices, such as Maria, were forbidden from singing in the Abbey. However, these rules did not necessarily apply to the sisters who are singing. In addition, it's a musical, and the nuns weren't "really" singing. Their complaints about Maria are simply put into a song.