Factual error: When they're making their escape in the Citrôen a shot is shown under the car. The tyres on the Citrôen are "Michelin X" radials. These were not on the market till 1948.
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.
Audio problem: When Georg playfully taps Max's cheek, we hear 3 taps but Georg only taps twice.
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.