Open Fields
We see the von Trapp children singing with Maria as she plays her guitar in the open fields near their home. The children have become alive and animated in their bodies and voices in singing with Maria. This is something they haven't experienced in some time as their father doesn't even call them by their own names; instead, he has a special whistle for each of them. Thus, the imagery shows us that the kids are coming back to life after their mother's death.
Crypt
The von Trapps hide in the crypt of the abbey in order to try to escape being found by the Brownshirts. The imagery tells us that they are close to death, as the Brownshirts have sided with Hitler's Nazi regime and won't take lightly to the Captain and his family attempting to run away.
Marching
We see the von Trapp children march into place when first meeting Maria. The imagery of children marching based on a whistle from their father, the Captain reveals a strange contradiction as the children are being led like a they are in the military though they are in their own home.
Maria
We see Maria singing in the open fields during the opening scene of the film. This imagery captures the largeness of feeling that Maria holds in her and is capable of expressing. Moreover, this imagery reveals the grand nature of this woman's soul and that she is meant for something greater in her life.
