Wirelessly posted (HTC Diamond: Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.11) Sprint MP6950SP)
Third device but the same operator.
Something is getting screwed up or interfereing with your sounds and notifications. Check and make sure nothing is disabled first. I recommend you use an app like sktools or cleartemp to clear out old temp files and clear notification que. Those leftovers may be causing your problems.