For now we just tried with a simulation on ISIS.
We still have to connect the microphone to the Arduino by amplifying the sound and adding a DC component.
Finaly, we used the library fix_fft : Modified 8bit FFT in c - Development - Arduino Forum and set FFT_N to 256 by using an Arduino Mega 2560.
I prefer wait Tuesday before to explain, I would have the code available to me and it will be simpler to explain