参考了一下 StackOverflow https://stackoverflow.com/questions/49140159/extracting-audio-from-a-video-file/49182456#49182456
以下代码可以提取出音频，但是音频体积却比源视频大的问题却没解决。
或许我该用 offlineAudioContext.startRendering() 这个方法试试？

var videoSrc = 'http://otof18y9e.bkt.clouddn.com/frag_bunny.mp4'

function startSelect(){
  var audioContext = new OfflineAudioContext(2, 44100 * 100, 44100);
  // var audioContext = new window.AudioContext()
  var videoFileAsBuffer = new Promise(requestVideo)
  videoFileAsBuffer.then(function (data) {
    console.log(data)
    audioContext.decodeAudioData(data).then(function (decodedAudioData) {
        mySoundBuffer = decodedAudioData
        soundSource = audioContext.createBufferSource()
        soundSource.buffer = mySoundBuffer
        soundSource.connect(audioContext.destination)
        // soundSource.start()
        console.log(mySoundBuffer.length)
    });
  })
}

function requestVideo(resolve){
  var xhr = new XMLHttpRequest()
  xhr.open('GET', videoSrc, true)
  xhr.responseType = 'arraybuffer' // 'blob'

  xhr.onload = function(e) {
      var binaryData = this.response
      // console.log(binaryData)
      resolve(binaryData)
  }

  xhr.send()
}

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后来我知道了，音频抽取后经过了解码，成了未压缩的PCM数据，体积当然很大。