All I can add is this ( maybe I'm lucky ). On iOS 8 as delivered, both phones (6, 6plus were fine - iOS 9 brought big trouble with the sounds/notifications, intermittently as discussed. I did 9.1 on both phones on iTunes and since then all has been fine again ( a week or so ) So every case seems different . I wonder if OTA updates don't fix it and full iTunes updates do ..... The update itself was quick and smooth, once the download was done which was really a trial and test of patience in both cases. Actually 3 cases , because my iPad mini was the same .... iOS 9.0.1 and 9.0.2 were very buggy with sound issues, iOS 9.1 has fixed it , and was again, an iTunes update using the iMac.