If you get muffled sound, you most likely do not have any problems with the flex cables. This might mean there is dirt somewhere, or you have faulty mic settings.
No sound at all points to a broken mic or damaged flex cables. If you (or anybody else) have never opened up the device you do not have a broken flex. They don't break on their own.
Buzzing noise might mean damaged mic (water, moisture...) or some faulty mic settings. For some people, disabling mic noise suppression and equalizer have helped.