Join Google's Chris Wilson as he asks, and answers, "Why do we need another audio API?" While the audio tag is excellent for playing an audio file and streaming, it's limited in the sense that it wraps everything up in one step: loading, decoding, and getting ready to play.
The Web Audio API, however, provides us with:
1) precise timing of lots of overlapping sounds
2) an audio pipeline/routing system for effects and filters
3) hooks to analyze and visualize audio data on the fly
All the sudden a whole new world of audio possibilities opens up, like online digital synthesis and super precise audio triggering for games and other applications. Chock-full of demos, if you like the internet, music, and synthesizers, check out this great presentation.