Un esempio

codice:
<head>
<script type="text/javascript">
function preloadAudio() {
  var aFile = new Image();
  aFile.src = "path/al/fileaudio.wav";
  aFile.onLoad = download(); 
}

function download() {
   alert('caricato');  
}
</script>
</head>
<body onload="preloadAudio()">
...
</body>
</html>