you'll need to open the iphone to investigate further. It's only a tiny plastic tab.. it may have come out of it's groove or been worn/fractured with time and use. I'd take a look and see whether the screws are loose that hold it in,if that fails then I'd replace the flex +/- the outer button too just to make sure. The guide for headphone/volume/mute and power button is linked under the photo to the right. Good luck it's a PITA job. Otherwise you could wander round with a bit of blutak to keep the button in either mute or ring?