iPhone 4 people can't hear me after 10 sec or 2 minutes

I just updated version 6.1 for my iPhone 4 now I got this "people can't hear me after 10 sec or 2 minutes" problem, but when it happened I select Speaker then it work fine, so I turn off Speaker it happen again.

Seem like it only happen when I'm not using Speaker.

I researched for it, but no answers for this problem, my iPhone warranty expired, what I have to do right now?, does anyone have this problem before and known the way to fixes it? please help me.

Thanks in advance.