HTML5 track cues empty

Question

I am attempting to display subtitles for an audio recording using a webvtt <track> element like so:

<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>

Populating the display div with this script:

<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>

Yet, when I inspect the track in firefox, though it has a cue object, it is empty:

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

I checked, and I have the mime-type configured correctly in the server. What am I missing to cause it to not load?