If dialogue from the main actors is being recorded then no noise is made. Crowd noise is dubbed on after. Same with music being played in dance scenes, where dialogue needs to be heard from the foreground actors.
Extras are asked to mouth their "words" or speak in a whisper.
As an actor I can con confirm that the rhubarb issue is a bit of an urban myth - maybe it was the preferred method once upon a time but certainly not now. Any actor worth his salt who needs to appear to be talking, with nothing scripted, would speak (or mouth) in character ie. the content would be what the character would be saying at that moment.
What about the extras who are not supposed to speak (so they don't have to be paid I presume) managing to get a 'thank you' in when being served? I always think they are really rude when they don't answer whoever is serving them in a pub or a shop.