Maybe there's a slight misunderstanding here.
IMHO, the crowd shouldn't be shouting over the cars or anything, but I'm pretty sure people have conversations or what to speak to their buddy when there is a bit of time where no cars are coming by.
I a low murmur for a crowd in game. Speaking just loud enough to be heard by the person next to them over the others talking. Of course, the more people you have in that crowd, the louder the person must talk.
Also now that I think about it, with the types of mods I've seen around, another user pointed out that there is quite an extensive detail a person could go into with something like this. The events that can trigger responses insim are absolutely astounding.
I finally remembered it was my first sim, Nascar Racing 2003 Season that had the crowd noise I was thinking of. I merely played a crowd just before you got in the car for the race IIRC.