"Displaying the frequency" can mean many things. Actually, my PitchDetect demo DOESN'T use a Fourier Transform - it uses autocorrelation. But that will only give you a single pitch, at high accuracy. If your signal has multiple simultaneous notes - well, that's a hard problem.
If you want to see a frequency analysis breakdown of the live mic input, check out http://webaudiodemos.appspot.com/AudioRecorder/index.html (code at https://github.com/cwilso/AudioRecorder). That uses the built-in FFT in the RealtimeAnalyser to show a frequency spectrum graph of a live audio signal.