I have the same problem
Mines done exactly the same. I;ve had it since the 11th july and all has worked fine until two days ago. My handsfree doesnt work anymore. Nobody can hear me on the microphone and it doesnt skip songs anymore. I tried my friends headphones and still the same problem so its a handset issue.
If you look closely my headphones dont seem to be flush with the iPhone anymore, its as though something is blocking it, but theres nothing there.😕