// Create an <audio> element dynamically.
var audio = new Audio();
audio.src = 'myfile.mp3';
audio.controls = true;
audio.autoplay = true;
document.body.appendChild(audio);

var context = new webkitAudioContext();
var analyser = context.createAnalyser();

// Wait for window.onload to fire. See crbug.com/112368
window.addEventListener('load', function(e) {
  // Our <audio> element will be the audio source.
  var source = context.createMediaElementSource(audio);
  source.connect(analyser);
  analyser.connect(context.destination);

  // ...call requestAnimationFrame() and render the analyser's output to canvas.
}, false);

http://updates.html5rocks.com/2012/02/HTML5-audio-and-the-Web-Audio-API-are-BFFs

createMediaElementSource()

  var source = context.createBufferSource();
  source.buffer = buffer;
  source.connect(context.destination);
  source.start(0);

AudioBufferSourceNode

  request.onload = function() {
    context.decodeAudioData(request.response, function(buffer) {
      helloBuffer = buffer;
    });
  }

oscillator = context.createOscillator();
oscillator.type = 'sine'; // square, sawtooth, triangle
oscillator.frequency.value = 1000; // value in hertz
oscillator.connect(context.destination);
oscillator.start();  

OscillatorNode

navigator.getUserMedia({audio: true}, function(stream) {
  var microphone = context.createMediaStreamSource(stream);
  var filter = context.createBiquadFilter();

  // microphone -> filter -> destination.
  microphone.connect(filter);
  filter.connect(context.destination);
}, errorCallback);

MediaStreamAudioSourceNode

 filter = context.createBiquadFilter();
 filter.type = "highpass"; // lowpass bandpass lowshelf ...
 source.connect(filter);
 filter.connect(context.destination);

BiquadFilterNode

http://caniuse.com/#feat=audio-api