A sound effect for the murmur of a crowd in the background.
Walla is often use as subliminal aural communication and  sets a mood or a tone.

The word walla was created in the old radio days when they needed the sound of a crowd in the background. They found if several people simply repeated "walla, walla, walla, walla"  it sounded like people talking. The audience did not really hear the words, just the buzz of voices.

Walla group (another term is ADR group) 
Today the walla group  use real words and real conversations. The walla actors come prepared. They have been informed of the period and locale of the film and have researched the local jargon and geography so that the background dialogue will be authentic. The material is mostly improvised. Group walla  has to be cut very skillfully like sound effects so that it does not sound artificially placed. The audience do not what to hear too much every time the principal actors pause. The editor weave the group walla in and out. 

It is usually the ADR supervisor who work with the walla group. At this point of the postproduction shedule the director might need to be on the scoriing stage woth music or in the cutting room with opticals.

The sound coming from a radio or loud speaker are mostly written and recorded by the ADR group. This because If in production they use a real radio or TV program then they will have the problem of getting legal clearances and perhaps paying fees.

There are no group sound on the production soundtrack. The extras do not say anything - they are mouthing it.
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>> READ What is a WALLA group?
WALLA group thread feb 2004 Cinema Audio Society webboard

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