A toy made from combining Daniel Shiffman's "Boxes" demo from his pbox2d library and the "Input FFT" demo from Krister Olsson's ESS library.
You can use your voice to control the spectrum bars, which interact with the box. You can add more boxes to the scene by clicking the mouse. Since it uses microphone input, you'll have to accept the initial Java security warning, but hopefully it shouldn't be problematic after that. Enjoy!