Here’s a basic Fast Fourier Transform implementation that’s kind of similar to the fmod one in the soundPlayer except that you can use it on incoming audio among other things.
I’ve made a dirty C++ wrapper for Dominic Mazzoni’s FFT code that is used in Audacity among other things. I believe this code is GPL.
As there are a bunch of general FFT related functions (including inverse FFT) this class could be used for all sorts of things…but in the example it just operates on incoming audio to create a spectrogram.
All you need to do is compile the application and send some audio into the record input of your default sound card. So a microphone is needed.
download the code (based on V0.02): http://www.digitalstar.net/openframeworks/audioInputFFTExample.zip.