javascript - Load audio file with PHP and play with JS

I am trying to load a wave audio-file with php and play the audio with javascript.

My Javascript code:

var xmlhttp = new XMLHttpRequest();
xmlhttp.open('POST', 'php/test.php', true); 
xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xmlhttp.onreadystatechange = function(){
  if(xmlhttp.readyState == 4) {
    var audio = new Audio("data:audio/wave;base64," + btoa(unescape(encodeURIComponent(xmlhttp.responseText))));
    audio.play();
  }
}
xmlhttp.send(url);
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My PHP code:

<?php
  readfile("test.wav");
?>
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Unfortunately it's not working. Any ideas?