STFT overlap add method - What am i doing wrong?
I think, the problem lies here:


  
  scale back as a result of the FFT
  clear non overlapping area in innput/output signal mic.
  


Why are you doing this? You are using a Hamming window with 50% overlap, summed up, this yields a constant one. If you delete the "non overlapping" part, you effectively change the window you are using to a weird "half hamming" one, this is bound to yield incorrect results. Skip these two steps, and you should be fine.