In the world of sound design, sweetening refers to the reinforcement of a sound to convey the most effective, emotive representation possible. Sound sweetening is identifiable in every modern multimedia format that includes sound design. The most common method of sweetening a sound comes from layering sounds to heighten it has on the audience and in modern media formats this is applied without restraint. For example, almost every sound in mainstream film today, from the closing of a car door to the exploding of a helicopter is not an accurate representation of the sound but instead a created detail to immerse the audience. A lot of sound sweetening reacts with subconscious cognitive responses, relating a sound to a known reaction of the human psyche. I.E animal roars within fire to create fear.
Sound sweetening effectively makes on screen sounds larger than life, often enhancing the bottom end to provide more oomph.
Jordan Vaughan: Creative Sound Design 