How do I do it? I already have notifications so if someone contacts me I already know because there's a pop up notification. I don't need an annoying sound to notify me as well, how do I turn it off?
I found out how, if anyone runs into this problem go to Messages, select Preferences and navigate to the Alerts tab. In the drop down menu options as needed and uncheck the sounds.
Big derp on me missing it, but here's what you should see.