The Sound of Music is the ultimate family-friendly movie. There's no cussing, no violence, and aside from some brief kissing (between the captain/Maria and Liesl/Rolfe), no hanky-panky. It's really all pretty G-rated…that is, until you get to the Nazi stuff.
The movie doesn't focus too intently on the Nazis and what they're about, but as an audience, we're very aware that World War II is about to erupt and Austria is gonna get trampled under Nazi boots. Guns are pointed, threats are made. That's definitely the kind of stuff you'd want a parent around to contextualize, so we'll bump it up to PG on that basis even though MPAA gave it an unadulterated G.