Walla

Background ambience or noises added to create the illusion of sound taking place outside of the main action in a picture.
Found on http://www.filmland.com/glossary/Dictionary.html#A

Walla

In American radio, film, television, and video games, walla is a sound effect imitating the murmur of a crowd in the background. A group of actors brought together in the post-production stage of film production to create this murmur is known as a walla group. According to one story, walla received its name during the early days of radio, when it ...
Found on http://en.wikipedia.org/wiki/Walla
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