Robert Wise's approach was that at the beginning of the film Maria and the nuns would sing because that was natural for them. When Maria teaches the Von Trapp kids to sing, they naturally become participants in the singing ensemble. The captain joins the singing group when he falls in love with Maria.
In Wise's approach, there is no reason why Max and the Baroness should be singing. Rolf gets to sing (I guess) because he's in love with Liesl.
