In the case of a 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 musicians Simon and Garfunkel.
The original acoustic version of the song features just their vocals with one guitar.
[1] 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.
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.