canned laughter
canned laughter {n.}, {informal}
The sounds of laughter heard on certain television programs that were obviously not recorded in front of a live audience and are played for the benefit of the audience from a stereo track to underscore the funny points.
"How can there be an audience in this show when it is taking place in the jungle? — Why, it's canned laughter you're hearing."
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Source: A Dictionary of American Idioms