Currently (February 2011) the Web Audio API only works in patched versions of Chrome and Safari. You can download builds of these patched browsers here.
The Web Audio API is currently under heavy development and this is just a first rough example that may at any time in the future break on new browser builds. Here is the current specification proposal by the W3C Audio Incubator Group.
The music track for this visualization is an excerpt from "Truth 2" by Jon Gillham and is used with permission by the artist. Thanks, Jon!
You may also be interested to see some of my other explorations of new HTML5 features here.