“Walla” is usually recorded later in a sound studio as part of post production and generated by a “loop group” of actors

Kia ora maelduin.

There were other words that these 'extras' employed as well such as 'Natter natter' or Blah blah blah! When said in chorus its generally undecipherable Natter.

It is quite common nowadays to utilise digital effects libraries to build a soundtrack. Canned laughter is similar in the sense that there is quite a range of laughter types from boisterous to, haha, I got it chuckles. I don't know about your part of the world but 'Extras' down here are on occasion referred to as 'Warm Props'