You might have seen that the new Angry Birds for Chrome has audio support but it falls back on Flash. Boo. Cheating.
Also found this SuperCollider-like browser-based audio API: https://github.com/colinbdclark/Flocking
You can follow the emails from the W3Audio Working Group here: http://lists.w3.org/Archives/Public/public-audio/ – they make for some fascinating insight into how these APIs are created, and what goes into creating an audio & video API from scratch. They also have a nice Twitter feed for keeping up on what’s going on with audio support: http://twitter.com/#!/w3caudio
From the feed I found these handy links for low-level audio API goodness:
There’s some Javacsript for handling music theory (Western, anyway) by calculating note frequencies, scales, octaves, transposing, etc:
There’s also audiolib.js:
It looks promising, but the bells-and-whistles example (with the sequencer and keyboard) didn’t work in FireFox 4.0 for me.