I have an iPhone 8 plus in a mophie holster. When I had this same holster and an iPhone 6s, alert sounds could be heard in the holster but not the reading of those alerts. I wrote to apple accessibility and was told that it was a problem with the phone sensor at the bottom of the holster. Sometime after upgrading to iOs 11, I could magicly hear again those alerts. But, with the upgrade to the 8 Plus those alert readings are once again silent as long as I have the phone in the holster. Does anybody know if Apple is aware of this problem and if they are taking steps to correct it?
My twin brother has the iPhone 8 and is experiencing the same problems.