The "Music" in The Sound of Music isn't just there for sing-alongs. It's really a symbol of all that's good and joyful in the world. Which means "lack of music" is code for sadness and despair.

We learn almost immediately that Maria has an irrepressible zest for life, and it comes bursting out in the form of music. She's singing when we meet her, unable to contain her joy at being outdoors on that beautiful day…and that's pretty much the case throughout the film.

The captain, in contrast, starts out being all about rules and keeping his distance from the people around him—and this includes a strict "no singing" rule for the kids (and himself, too). It appears that when his wife died, he walled himself off from love and joy, and music went out the window with them.

When Maria gets her happy little hands on the kids, that changes big time. The captain realizes that he should let both love and music back into his life. His first real moment of connection with the kids in the film occurs when he hears them singing to the baroness He dramatically joins in, shocking them with his awesome pipes—and warmth. They hug after the song, signaling that the captain's relationship with his kids is undergoing a major thaw.

When the captain later thanks Maria for bringing music back into the house, he's also thanking her for bringing love back into his life. Music is a huge vehicle for conveying emotions and forging connections to other characters, so it's a lot more than just song and dance.

And does Herr Zeller ever sing? Case closed.