html5轨道提示空

Question

我试图使用webvtt元素显示音频录制的字幕,如下所示:

<audio controls="controls">
      <source src="TheBrigands.mp3" />
      <source src="TheBrigands.ogg" />
      <track default="" kind="subtitles" srclang="en" label="English text" src="TheBrigands.vtt" />
</audio>
<div id="display">
      It was a cold and stormy night,
</div>

使用此脚本填充显示div:

<script>
  function main() {
     var recording = document.querySelector("audio");
     var track = recording.textTracks[0];
     var display = document.getElementById("display");

  track.mode = "hidden";

     track.oncuechange = function () {
        var cue = this.activeCues[0];

        display.innerHTML = cue.text;
     }
  }

  window.onload = main;
</script>

然而,当我在firefox中检查轨道时,虽然它有一个提示对象,但它是空的:

document.querySelector('track').track.cues
TextTrackCueList { length: 0 }

我查了一下,我在服务器中正确配置了mime-type.

我错过了什么导致它无法加载？