I've been struggling with this on an iPhone 6s 64GB. I originally noticed this after upgrading from 10.2 to 10.3 Beta 1. I have my phone integrated with my Mac for phone and messages. If I have iMessages open on my Mac but inactive (in the background) and a text message arrives, my phone sometimes stays silent. Upon activating the iMessages window, my text alert goes off on my phone and stutters, almost as if the phone is attempting to replay the sound over and over. I've tried wiping the phone and restoring from backup to no avail. I then wiped the phone to the 10.3 image using iTunes. At that point, I installed my apps manually and reconfigured everything manually. Still, the issue persists. Unless a bug of the first 10.3 beta can somehow survive these two restore operations, I doubt the first beta did any permanent to cause it. Any idea how I can troubleshoot this?