It's known that to give the audience the impression of a lively talk in crowd scenes (as a background noise), actors mumble something. For example, in Russia - it's widely known here - they may reiterate the phrase which may be translated into English as "What we should say is that there's nothing to say" or something alike.
It goes without saying that something like this exists in the English language and is exploited by English-speaking actors, and I'd very much like to know what it is.