Talk about a no-brainer.

Our protagonist uses music to transform the von Trapp household, so it seems right that the sound of music would take top billing in the title. It's also very meta, since the film is a musical and it's about a family who became a musical performing bunch.

BTW, the original working title of the stage musical was Love Song. Rodgers and Hammerstein's lawyers begged them to change it because of copyright problems with the 300+ other musical compositions with the same title (source). We're so glad they did.