“...responsible for everything the audience hears. Exactly what that entails can vary considerably, depending on the type of show, the performers in the show, and the performance venue.” -- ASDP
When sound designers start their work, their objective is to have the audience's emotional and dramatic connection with the performance be what they are aiming for.
They'll work fairly closely with the director while creating their sound design, creating a natural and abstract world for the show's story.
Most/some sound designs will include effects, atmospheres, sonic textures, and filmic ambiances. Sound designers also must design a sound system for the specific production and auditorium that will give the audience the best experience of the show.