webrtc 屏幕共享时添加声音

navigator.getUserMedia = navigator.mozGetUserMedia || navigator.webkitGetUserMedia;
  navigator.getUserMedia(screen_constraints, function (stream) {
    navigator.getUserMedia({audio: true}, function (audioStream) {
      stream.addTrack(audioStream.getAudioTracks()[0]);
      var mediaRecorder = new MediaStreamRecorder(stream);
      mediaRecorder.mimeType = 'video/mp4'
      mediaRecorder.stream = stream;

      document.querySelector('video').src = URL.createObjectURL(stream);
      var video =  document.getElementById('screen-video')
      if (video) {
        video.src = URL.createObjectURL(stream);
        video.width = 360;
        video.height = 300;
      }
    }, function (error) {
      alert(error);
    });
  }, function (error) {
    alert(error);
  });
});