In sound design, sweetening ("to sweeten") refers to "juicing up" the audio portion of a film, play, computer game software or any other multimedia project. Its origin may have been old-time radio, which produced visual detail with sound effects such as people walking, horses galloping, doors opening and closing, gunshots, "body slams," etc.
In the case of music performance or recording, sweetening may refer to the process of adding instruments in post-production such as those found on The Sounds of Silence by folk troubadours Simon and Garfunkel. The original acoustic version of the song features just their vocals with one guitar. Producers at Columbia Records, however, felt that it needed a little spicing up to be a commercial hit, and so without the consent of the artists, they added drums, electric bass, and electric guitar.
In television, sweetening refers to the use of a laugh track in addition to a live studio audience. The laugh track is used to "enhance" the laughter for television audiences, sometimes in cases where a joke or scene intended to be funny does not draw the expected response, and sometimes to avoid awkward sound edits when a scene is shortened or more than one take is used in editing. Sweetening has been used in a number of television series, from older shows like Happy Days, Taxi, The Mary Tyler Moore Show, to newer sitcoms Two and a Half Men and 2 Broke Girls. The act of sweetening is demonstrated in the Woody Allen film Annie Hall when Alvy Singer visits his friend Rob, played by Tony Roberts, in Los Angeles. At one point, Rob has the engineer add laughter to cover voiced disapproval from the audience. Some shows used the canned laughter technique very obviously rather than the "in-between" technique (a recording from an external audience, but genuine laughter) described as a laugh track. An obvious sign of this is that the laughter is more or less identical in volume or magnitude, regardless of how extreme the joke is.
In bigger music TV shows, sweetening is used to enhance the sound of the visible audience. It is often difficult, for a number of reasons, to pick up the sound of the real audience, so audio sweetening is used so that viewers hear what they see - an engaged audience.