Sound design encompasses all audio in a video (atmospherics, foley, sound effects) everything except for the music (okay, so it does sort of include music – more on this here). Think of sound design as the auditory texture of a video – in the same way that lighting and colour are the visual texture. Like light and colour, sound is a big part of telling a story.
Nevertheless, sound design is without doubt the most neglected element of video production. For some unfathomable reason, content producers too often treat sound as an afterthought. A dire error seeing as bad visuals and good sound are a lot more tolerable than great visuals with bad sound. In fact, even with the most pristine visuals, a video will be unwatchable if the sound quality is bad.
Poor sound design makes video look amateurish. Obviously, this undermines the message. Good sound design on the other hand punctuates the visual elements and gives them added resonance.