通过PHP向移动设备提供mp3

I have a very simple PHP script that serves an mp3 file back to the browser. It works perfectly on desktop browsers, but unfortunately doesn't play on any of the mobile devices I've tested (Android Phone, iPad Mini). Can anyone see what I'm missing?

<?php
    $file = 'track.mp3';

    header('Content-Type: audio/mpeg');
    header('Content-Length: ' . filesize($file));
    header('Cache-Control: public, must-revalidate, max-age=0');
    header('Pragma: no-cache');
    header('Content-Disposition: inline; filename=track.mp3');
    header('Content-Transfer-Encoding: binary');

    readfile($file);
    exit;
?>

Please note: I'm aware that this is pointless in and of itself; this is a simplified version of a much larger application, but I've stripped out the rest of the code to provide the simplest reproducer I can.

Thanks a lot for your help!