Hello,
Take a look at the example I uploaded to "Share Your Creations" sub-forum about a month ago. It's called "FFT Test".
If you want to look at the frequency of audio captured by the microphone, you'll probably end up doing something like:
1) Acquire audio records from the microphone.
2) Perform an FFT and square root of the magnitude squared to obtain the "spectral power" of the audio.
3) The "spectral power" of the most abundant frequency will be represented by the "bin" with the largest value. Each bin represents the frequency range of the Nyquist frequency divided by the number of bins.
My example uses a 64 point FFT, but I have also used this example to do a 256 point FFT.