OfflineAudioContext

// define o contexto de áudio online e offline

var audioCtx = new AudioContext();
var offlineCtx = new OfflineAudioContext(2, 44100 * 40, 44100);

source = offlineCtx.createBufferSource();

// usa XHR para carregar uma faixa de áudio, e
// decodeAudioData para decodificar e OfflineAudioContext para renderizar

function getData() {
  request = new XMLHttpRequest();

  request.open("GET", "viper.ogg", true);

  request.responseType = "arraybuffer";

  request.onload = function () {
    var audioData = request.response;

    audioCtx.decodeAudioData(audioData, function (buffer) {
      myBuffer = buffer;
      source.buffer = myBuffer;
      source.connect(offlineCtx.destination);
      source.start();
      //source.loop = true;
      offlineCtx
        .startRendering()
        .then(function (renderedBuffer) {
          console.log("Rendering completed successfully");
          var audioCtx = new (window.AudioContext ||
            window.webkitAudioContext)();
          var song = audioCtx.createBufferSource();
          song.buffer = renderedBuffer;

          song.connect(audioCtx.destination);

          play.onclick = function () {
            song.start();
          };
        })
        .catch(function (err) {
          console.log("Rendering failed: " + err);
          // Nota: A promessa deve rejeitar quando o StartRendering é chamado uma segunda vez em um OfflineAudioContext
        });
    });
  };

  request.send();
}

// Run getData to start the process off

getData();