I had the the same problem already twice and was able to solve it. In my case two tiny connectors of the main flat cable were broken and hence no connection to the normal "lower" microphone could be established. After some seconds, the phone switches to the external microphone, which is quite inconvenient for the person you are talking to because it is quite noisy and catches sounds from the environment. The connectors are supposedly broken because my phone fell down multiple times. Be sure to check for the remaining broken pins in the female plug on the main board and remove them.