A discussion in the radiodrama e-mail group provided some interesting information:

Tony Palermo opened a discussion on Walla. ...the atmospheric mumbling known as Walla Walla. It's cheap and incredibly evocative in conjuring up barrooms, quilting bees, legislatures, pirate ships, etc. I create convincing niteclubs with a few clinking glasses and plenty of cocktail walla. When working with large casts in live radio productions, I find I often have to tone down the walla walla because "a little goes a long way." In my scripts, I indicate walla instructions as a sound effect cue. When directing, I lead the walla myself. Working with student groups I have to remind them that good walla requires non-distinct mumbling punctuated with a few real words--but nothing that would steal the audience's attention away from the scripted dialogue. Kids will sometimes just mumble the words "Walla Walla Walla Walla"--which is cute, but no good in pulling off the correct illusion.     TP Other comments include: ...over at Jabootu, they call this "watermelon, watermelon, cantaloupe, cantaloupe" and I recall David Gerrold describing the sounds as "natter, natter, grommish, grommish." Does anyone else use any different "words"? in the UK, we call this sort of effect .. "Rhubarb" as in a crowd standing around muttering "rhubarb, rhubarb rhubarb.... mind you there's always some clown who will add, "custard" but providing it's muttered and kept low, seems to work for us. DR A professor taught us to say "peas and carrots" over and over along with "ugamumble - ugamumble" -- this sounded surprisingly like conversation in the background if half were doing peas and carrots and half were doing the ugamumble parts. SZ I dislike using random chatter. I prefer having my actors work out little scenes for themselves, creating lives for their "crowd characters" that match what is going on in the play and letting them mill about in the studio and record this. JB In Germany, we used the German word for what they use in the UK, "rhubarb," in German "Rhabarber, rhabarber, rhabarber." Works even better in German because of the added syllable, and the guttural R simulates some of the northern German (educated vis-à-vis the dialect of other regions) speech rhythm. JH For courtroom walla walla,... going to my local RC church. The crowd's rustling and subdued coughing, the shuffling of arriving parishioner. What I had then was 3-4 minutes of increasing courtroom walla walla. I also had the rising of the congregation. JH A potential problem to watch for is if you use the same actors for walla that you use for your primary voices, the individual voices may be recognizable. For small groups recording their projects, you may want to team up with another theater to provide the walla.