Mute Notification Center Alert Sounds

Notification Center is a great addition to OS X Mountain Lion but the alert sounds can be pretty annoying if you have a lot going on. Rather than muting all system audio to hush the constant chiming, you can directly silence notifications on a per-app basis.
  1. Open System Preferences from  Apple menu and click the “Notifications” menu
  2. Select apps from the left and uncheck the “Play sound when receiving notifications” box
  3. Repeat for every app you want to silence