<audio id="myAudio" controls oncanplay="myFunction()">
  <source src="https://download.samplelib.com/mp3/sample-3s.mp3" type="audio/ogg">
  <source src="https://download.samplelib.com/mp3/sample-3s.mp3" type="audio/mpeg">
  Your browser does not support the audio element.
</audio>

<script>
function myFunction() {
  alert("Can start playing the audio");
}
</script> 

<video id="myVideo" width="320" height="176" controls oncanplay="myFunction()">
  <source src="https://www.sample-videos.com/video321/mp4/240/big_buck_bunny_240p_20mb.mp4" type="video/mp4">
  <source src="https://www.sample-videos.com/video321/mp4/240/big_buck_bunny_240p_20mb.mp4" type="video/ogg">
  Your browser does not support HTML5 video.
</video>

<script>
function myFunction() {
  alert("Can start playing video");
}
</script>