This is the only thing I can think of. I resorted an iPad air 2 from an iPad mini 2 that had the mute switch on. I now have no notifications sounds at all. I have checked do not disturb, all of the notification sounds, and did a reset holding the home and power button. Has anyone else ever has this problem? There is no mute switch on the air 2, and I think the backup somehow picked up the mute switch if that makes sense.