<?php 
$kelime = '<a href="001.mp3" >';
$kes = explode('<a href="',$kelime);
$kes = explode('" >',$kes[1]);
echo $kes[0];
?>

Sonuç : 001.mp3

<?php 
$kelime = '<a href="001.mp3" >';
preg_match('#<a href="(.*?)" >#si',$kelime,$icerik);
echo $icerik[1];
?>

Sonuç : 001.mp3