David Humphrey’s latest patch (which we are currently testing) implements the FFT in C++ within Firefox and reveals the spectrum data to js. The hope is that this frees up more cycles for animation/other calculations on the js side.
We are currently discussing which features we would like added to the API. If you would like to join in on the discussion, we hang out in #processing.js / irc.mozilla.org
That being said, audio programming is way behind the rest of the RAD movement. ChucK is the only thing close to RAD audio programming, but the project is run academically.