It seems to me that the notifications volume is turned up, you can check the status by using the small Minix handset that came with your units, the easy way to check is to ensure that there is no audio or video playing and then press the volume plus on the handset .. this will bring up a volume at the top of the screen indicating the status of the notification volume the Icon should be a "Bell" ... I expect from what you say that it will be advanced toward the full volume ... now hold down the minus button and you will see the volume decrease ... If you are able to go into the setting menu and choose advanced settings you will be on a white background screen with areas of control placed neatly relevant to the usage access.... there is one for Sound .. so take a look in there and adjust the volume sliders to suit your needs ... that should solve the issue ... but you maybe are accidently turning up the Notification in the way I have described above ...
