Chatterer starts to play sound when all science already sended to KSC.
More fun is to start play sound when data sending is started, and stop play sound when data sending is finished\interrupted by lack of EC

Nice suggestion, thanks.
At the time I implemented this feature there was no way to know when a science transmission has just started or just ended, so I had to tweak a bit.
Maybe now there are some more "game events" trigger to look at to do this in a better way, I will give it a look.