Instant karaoke track with the Web Audio API

gistfile1.js

var url = 'http://static1.kevincennis.com/sounds/callmemaybe.mp3'
  , audio = new Audio(url)
  , context = new webkitAudioContext()
    // 512 samples per frame, stereo input, mono output
  , processor = context.createJavaScriptNode(512, 2, 1)
  , sourceNode

audio.addEventListener('canplaythrough', function(){
  sourceNode = context.createMediaElementSource(audio)
  sourceNode.connect(processor)
  processor.connect(context.destination)
  audio.play()
}, false)

processor.onaudioprocess = function(evt){
  var inputL = evt.inputBuffer.getChannelData(0)
    , inputR = evt.inputBuffer.getChannelData(1)
    , output = evt.outputBuffer.getChannelData(0)
    , len = inputL.length
    , i = 0
  for ( ; i < len; i++ )
    // flip phase of right channel
    // http://www.soundonsound.com/sos/sep04/articles/qa0904-7.htm
    output[i] = inputL[i] - inputR[i]
}