I had same problem, after lots of investigation and posts suggestions, it was an hardware issue, you need to change your Mic Strip, spear phone mic is different then what is there at the bottom. There is no software issue, I had formatted, upgrade to Latest Kitkat and then lollipop but nothing helped, the problem seemed resolved for some time after each upgrade, but only for a day, finally after the hardware change it got fixed and did not reoccur.