When you receive messages, a little noise is heard.
That icon gives you the ability to turn it on/off by clicking it.
It means the noise when someone sends you a message in chat. It's a little "bing" noise
It's talking about the noise the chat makes when someone sends you a message.
You can choose if you want a ring sound or a pop sound when you recieve a message. That kinda thing.
Dumb question... It's obvious what it is.