When sounds in the background are captioned, they come forward. All sounds become equally “loud” on the caption track.
Source: The Happening, 2008. DVD. The featured caption is an example of backchannel speech sounds that come forward when captioned: “[Man] I just walked down a quarter mile. It was clean.”
Source: The Happening, 2008. DVD. A compilation of backchannel sounds that come forward and compete with the foreground speech when captioned verbatim.