Interpreting microphone output signal I have a question about interpretation of the signal produced by a microphone.

The output of a microphone is a voltage level that corresponds to the pressure on the membrane. The part that confuses me is why are the output signal values located above and below the average (picture attached). Do the values "above and below" correspond to the membrane deflecting, and then going back to its original position? If that's the case, should I just concider the values above the average for my further computation of the Fft? 

Thank you!
Sound behaves (more or less) like a wave. When the positive peak hits the membrane it is moved in one direction and afterwards "sucked" in the other. Just as waves on the beach do. 
Each positive peak generates a equal negative peak (in a perfect world...). Superimposition of different waves makes it hard to see this directly.

The average is not necessarily the atmospheric pressure, but is a combination of it in combination with the property of the electrical circuit of the microphone.

For your fft you have to consider positive as well as negative values. However, you may want to get rid of the average value if you do not care for very low frequencies.