Yesterday (April 13th), about 100 extras gathered in Atlanta and recorded multiple crowd style audio segments for use in the movie. Some of them included: "No more games!" and "Katniss! Katniss!"
This news comes from Jeffrey West, a blogger who took part in the recordings. (See end of post for his video, including some audio and video from the day!)
"I spent all-day Saturday in Atlanta doing some post-production audio work on the Catching Fire film. Basically we were running around screaming, and/or clapping, and/or laughing and straining our voices in every single way possible for about 8 hours."
Thanks to Jeffrey West and Hunger Games Examiner