I believe it's akin to the old iPad 2nd generation volume button problem. If you've ever torn a power/volume flex on an iPad 2 (I know I have), you may have over tightened the screws holding the volume/switch button assemblies. It locks the outer plastic volume button in place, and you have no volume control. I tried doing 4 different iPhone 5S's, using an Apple torx spec'd screwdriver, and found that I could take the SAME phones, tighten them by hand (with and without the plastic rings), and I lost the sound every time. I used my torx by hand (with and without the rings), and found that with they worked every time. You HAVE to be careful with over tightening things in Apple devices. Before, I was just putting the bracket screws in tight as they'd fit. Have you ever opened up an iPhone 5 and seen a loose bracket screw just laying in there, possibly dismissing it as a repair gone wrong? I think it's that they have to have some play.