Missing my MP3 Files
This code would be inserted into the primary web server's 404 error HTML file:
<body onload="missingMP3()">

<script>
function missingMP3()
{
  var _loc="";
  var _u=document.location.toString(); /* what is the bad URL? */
  var _a=_u.indexOf("/previews/")      /* does the URL contain this path? */
  var _b=_u.indexOf(".mp3");           /* does the URL contain .mp3? */
  var _c=_u.length-".mp3".length;      /* what is the length of this bad URL */
  
  /*
   * If the URL contains the path 'previews' and
   *   it also ends with '.mp3' then....
   */
  if(_a!=-1 && _b==_c)
  {
    /*
	 * ...then pick off the missing mp3 file and send the user
	 *   to the other location of the mp3 file.
	 */
    _loc=_u.substr(0,_a)+"/previews2/"+_u.substr(_a+10,_u.length);
	document.location=_loc;
  }
}
</script>