Not quite sure what you're asking, but I'll give a response. The "Default Notification" sound is pretty much exactly what it purports to be. Whenever a notification comes in for any app or service, if that app or service hasn't specifically chosen a notification sound other than the default, then the default notification is what you will hear.
So you can change the default notification sound by going to Settings -> Sound and Notification -> Ringtones and Sounds -> Default Notification Sound, and then selecting the new sound that you wish it to be.
99% of the time, apps or services will give you the option within their own settings to change the notification sounds specific to that app/service only. Sometimes they will have already incorporated their own custom sound as well.
If you're constantly getting notification sounds when no new notifications are coming in, in my experience the likely culprit (and I'm just guessing here based on previous experience helping others) is that you have a "nag me" app installed that constantly reminds you that you have unaddressed notifications.