利用PHP通过apache下载wav音乐文件无法打开文件

当我用PHP下载服务器上的wav音乐文件时，wav文件能够下载下来，并且大小相同，但是却不能播放。但若是通过文件服务器拷贝到本地，是可以播放该文件的，不知道如何原因。通过MD5sum检测，该文件下载后已改变，不知道在哪里被修改，请教大家，谢谢！
PHP code:

<?php
include ("./lib/defines.php");
include ("./lib/functions.php");
include ("./lib/database.php");

session_start();

getpost_ifset(array('confno', 'bookid'));

$query = "SELECT confOwner from " . DB_TABLESCHED . " WHERE bookid=?";
$data = array($bookid);

$result = $db->query($query, $data);
$row = $result->fetchRow();
if ($_SESSION['auth'] && ($_SESSION['privilege'] == "Admin" ||
$row[0] == $_SESSION['userid'])) {
if (is_numeric($confno) && is_numeric($bookid)) {
$file = "meetme-conf-rec-".$confno . "-" . $bookid . ".wav";
$playfile = "/var/lib/asterisk/sounds/" . $file ;
$mimetype = "audio/x-wav";
$content_len = filesize($playfile);
header("Content-Type:".$mimetype);
header("Content-Transfer-Encoding:binary");
header("Content-Length:".$content_len);
header("Cache-Control:private");