One of the new features in OS X Mountain Lion is the notification center. However, many of us just find the sounds made by it plain annoying. Fortunately, there is an easy to disable it.
* Open System Preferences on your Mac and click the Notifications menu.
* Now, select the apps from the left and uncheck the “Play sound when receiving notifications” checkbox
* Repeat for every app you want to mute. This will make sure you are not bothering your friends and people sitting around you.