Film sound is carefully designed and added during post-production to emphasize important elements and set the atmosphere. There are different types of sounds that serve different purposes - selective sounds highlight key elements while ambient sounds create a sense of place. Sound can also be diegetic, existing within the film's world, or non-diegetic from outside the world, like music or narration. During mixing, sounds are made synchronous or asynchronous to images and dialogue to reinforce or alter their meaning. Elements like voiceovers, theme music, and musical scores are also used to guide the narrative or set mood, while silence can build tension.