They are just saying 'walla- walla"! This is called a “walla- walla scene” and it is used to make the extras look like they are talking to each other. The only thing they have to say is walla over and over again to do the trick.
If you want to make it more realistic just mumble animatedly reproducing the fluctuations of conversation without much use of real phrases. In the U.K. it’s called “rhubarb”, and in Germany it’s called a “rhabarber”.
People literally say these words to fake conversations.
Did you know that the average extra’s salary is $112 a day?