Web APIs for audio are under active development. The W3C has formed a working group where use cases and two specifications (Web Audio API and MediaStream Processing API) are being developed. Chrome and Safari are shipping experimental versions of the Web Audio API now, and Firefox will soon be shipping experimental versions of the MediaStream Processing API (it's available now in a special build). Firefox 4 onwards also includes the simpler Audio Data API. I believe all of these APIs provide sufficient functionality to build the audio portions of media playback with working audio-visual synchronization.