I repair iDevices for a living. When I first heard of this report I was thinking defective earpiece speaker possibly or perhaps a component on the proximity sensor assembly which links the earpiece speaker to the logic board, but then someone stated that they hear it with headphones on as well. If it's happening with headphones then this may just be a simple software patch to fix it (possibly an audio amplification/equalization tweak) ...but if not, it could be a serious board level hardware problem much like the iPhone 6 plus had with the touch sensor controller failures (aka. touch disease). Guess we'll know after the next iOS update.
I sincerely hope Apple provides a fix soon, unlike their behaviors in the past where some fixes are announced and they are never delivered.
It was years ago, had a Mac with dual processors and the released an update which disabled the proprietary CD / DVD drive, after a year of no update and unwilling to spend over $100 to replace when I could buy a PC DVD for less than $40, I upgraded to a PC so that would never happen to me again. As you may guess no more OS upgrades permanently disabling my PC's hardware.