Same thing happened on my L830. Messages were arriving but no notification sounds. Eventually I pulled the battery and left it out overnight. Put it back and notification sounds were almost back to normal. I say almost because custom ringtones don't work now. I set them but get random tones (but never the default tone ) instead. Delete them, reset them with different tones and still get random sounds. Also after downloading the last 2 updates to 100% the phone has frozen with just the Windows logo showing for hours. Pulled the battery for a few minutes, put it back and the updates installed with no problem. Gotta love Windows Phone!