This is not currently supported by HTML5 audio. However, new features are in the works which would allow for this kind of audio processing. I think modern javascript engines might actually be fast enough too, considering its pretty much all compiled to machine code now.
The new audio APIs are probably about six months away, and then there's no guarantee all browsers will support them (Internet Explorer is particularly slow and reluctant to adopt new standards). So you'll probably just have to wait it out for now I'm afraid.