Run a function when an HTML5 Audio Element has played a certain %

Is there any way to run a predefined function when an HTML5 Audio element has played a certain amount (such as 50%, 80%)? Let's assume I know the size of the file and anything else.

<div class="audio-player" style="display:none;">
  <audio id="music" preload="auto">
    <source src="/path/to/my.mp3" type="audio/mp3">
  </audio>
</div>

I'm already using Javascript to play/pause this audio.

function aud_play_pause(_id) {
  var audio = document.getElementById("music");
  if (audio.paused) {
    audio.play();
  } else {
    audio.pause();
 }
}

So let's say I had a function halfway() to tell if someone had listened to half the song, is there any way I can run this when the track reaches 50%? Even if this is an estimate, that's fine. I would also like to avoid the use of any frameworks, as I don't need any additional functionality.