It's very simple. First you should record or read the audio signal. For that you should do the following:
If you want to give the input through a microphone then you should use the command: wavrecord
If you want to give take the input from a .wav file you should use the command wavread
After doing that all you have to do is to use this command : fft
If you want to know the syntax of these commands see the matlab help. If you have any trouble in doing that notify me I will help.
Regards,
iVenky.