Visualizing the measure of each FFT frame as an image in an audio signal i wrote a function in MATLAB which will divide a
audio waveform (a speech) into smaller windows with overlap .The output will be
a matrix X, each column of which contains the windows which i export.
Then i calculate the FFT of each column and i visualize the measure of each FFT frame as an image, using the "imagesc" command. I saw a shape that is symmetric
to the horizontal axis. Why i have this symmetry?.
Very simple: 

The Fourier transform of every real-valued signal is (hermitian) symmetric.

You can thus, for your real-valued signals, simply not visualize half of the FFT bins.