How do I repair/replace the Fairphone's microphone?

The Mic in my FP sends only noise, so I can do calls onlöy with a headset. Reboot (incl. battery removal) did once help for an hour or so, but thern the problem started again. Another reboot did not help. So probably there is a loose contact or the mic itself is broken. Unfortunately the FP site does not offer a new mic and there is also no repair guide for this part. Does anybody know how to repai it?