Funny - I turn the volume full down and have no problem. Your other choice might be to find its channel on the sound mixer of your computer (lower right corner if you're running Windows), select it then either turn that down or mute it.
IF you go to the lower right corner, where the little chat window resides, there is a little gear. If you click on that gear, you can disconnect from the chat room.
Turning the volume down is not always good because I may want to listen to something in another window however it looks like the disconnect is a viable solution.
Good advise all, but I just turn my sound on and off at will. "Ka ching" and other such sounds annoy me. Unless I need sound I run a silent computer.