Prova questo.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function checkExt(valore) {
 if (valore.toUpperCase().match(".(WAV|MP3)$") == null) {
   return false;
 } else {
   return true; 
 }
} // function checkExt(valore)
//-->
</script>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
alert(checkExt("kkklllslls.mp3.txt"));
alert(checkExt("kkklllslls.mp3"));
alert(checkExt("pppp.kkklllslls.wav"));
alert(checkExt("kkklllslls.wAv"));
alert(checkExt("kkklllslls.mp3.doc"));
//-->
</script>
</body>
</html>