When you receive a notification in Apple's Notification Center for Mountain Lion, the system will use the built-in "Basso" sound. If you do not want this sound to play whenever you receive a notification, then you have a couple of options.
The first is to disable the sound completely, which can be done on a per-application basis in the Notifications system preferences. Simply select an application and then uncheck the "Play sound when receiving notifications" option, which should silence it.
Unfortunately this is a relatively limited option and one that requires you to change it … Read more