I have exactly the same problem. It's still here since DP 8.1.
I have twitted to windows phone support but they said it's hardware related but I don't think so. However, I'm still using it in this condition.
Hi, i try the way you told, but keep give me error, get 1% then restart the phone. i have problem with my mic, since i did the upgrade to WP 8.1, but if there is any solution for the mic, i will keep on WP 8.1, do you know any solution? Thanks