Audio problem: During the bedroom storm scene singing "My Favorite Things", Marta is sitting next to Fräulein Maria and she gets wrapped up in the moment of Maria singing and mouths "feel so bad" - she's not supposed to be singing.
Audio problem: During the song "The Lonely Goatherd," Maria sings, "Men drinking beer with the foam afloat," Marta blows foam from the cups into Maria's face, and she coughs. You can see that Julie Andrews' mouth is not at all in sync with the coughing on the recording.
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.