Music is an art form and cultural activity whose medium is sound organized in time. The common elements of music are pitch, rhythm, dynamics, and the sonic qualities of timbre and texture.
Sound design is the art and practice of creating sound tracks for a variety of needs. It involves specifying, acquiring or creating auditory elements using audio production techniques and tools.
Sound design consists of creating a sonic pallet for the reality of the story, that means coming up with the sound effects which aren't foley (which is the recording of sounds like footsteps, clothes, most of the ordinary interactions with objects, etc, all of which is done in a foley recording studio by foley artists). Designing sounds can be achieved by recording new material, manipulating library sound effects to create new sounds and by inventing new and extraordinary sounds through synthesis techniques,
Blurring the lines between sound design and music is something that some directors are willing to experiment with occasionally...