OK, you will now get an audio notification whenever someone posts a message in the chat. However, it will do it all the time - not just when you have the window minimized. When you're actually chatting and you don't want any sound, just hit the chat's mute button. The behavior that you actually want (where it only creates a noise when the chat window is minimized) is not possible with our current chat software. It's remotely hosted and controlled, and I cannot modify it beyond the configuration parameters that are provided.
There is also a chance that most people won't like this new configuration, and won't like hitting the chat's mute button. Try it out for now and see what you think.