The mic is located in the same area as the ear speaker. If your screen has been replaced before on the iPhone 6, it may not have been lined up correctly. It's a very small whole left of the proximity sensor is the screen is faced down. The solution is to make sure the hole is in line, and that the 4 gold contacts on the ear speaker are making contact with the 4 gold contacts on the Front Camera/ear speaker flex cable. If that doesnt work, the flex cable needs replacing. If the sounds going through to the other person is just faint but coming through then it defiantly is just mic position. The flex cable may have been cut or bent to harshly too. If you look where the hole is supposed to line up with the mic, sometimes a screen assy have them to small, they are adhesive, I usually take the tip of an needle and just help open them a little bigger.