function randMp3(){
  $mp3 = glob('*.mp3', GLOB_BRACE);
  return $mp3[array_rand($mp3)];
}

$extension = "mp3";
$mime_type = "audio/mpeg, audio/x-mpeg, audio/x-mpeg-3, audio/mpeg3";
$file = randMp3();

if(file_exists($file)){
    header('Content-type: {$mime_type}');
    header('Content-length: ' . filesize($file));
    header('Content-Disposition: filename="' . $filename);
    header('X-Pad: avoid browser bug');
    header('Cache-Control: no-cache');
    readfile($file);
}else{
    header("HTTP/1.0 404 Not Found");
}