Storyboards are simple drawings done during preproduction and production as a guide for, and to begin a conversation about, the content of shots and scenes. There should be sound design storyboards too, because it would present an opportunity (and an obligation) to actually think about, discuss, and plan the way sound will work in each scene, and in the movie as a whole. It would happen early enough so that sound ideas could have an influence on the way scenes are blocked, lit, and paced, as well as where they are shot.