Continuity mistake: Before the Edelweiss song, the Baroness takes a glass to her lips, but a a frame later her hand is down.
Continuity mistake: When Rolf brings a telegram he extends his arm and immediately after, he lowers it. A frame later it's extended and he is lowering again.
Continuity mistake: While Liesel tells the nun that she wants to visit Maria, Friedrich and Kurt keep changing positions between shots.
Continuity mistake: When the ballroom dance starts, the Capt. and the Baroness greet the guests. The number of people walking down the stairs behind them increase, decrease or simply disappear between shots.
Continuity mistake: While looking at the front of the house at night time, you see there is a window above the door, which is lit from behind. From the inside of the house, the front door doesn't have a window but has wainscoting above it.
Continuity mistake: When the captain sings for the first time and hugs the children, Gretl is under his left arm. In the next shot, he raises his arm and she is nowhere to be seen. Then, when Maria motions for Gretl to give the Baroness the edelweiss, she is back under his arm.
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.