Just found that IE 6/7 is unable to download mp3 files over HTTPS while we were setting:
header("Cache-Control: no-cache, must-revalidate");
Firefox was downloading files ok, while IE 6/7 would pop open the save as… dialog, and sit at 0% downloading. Once we changed the headers to:
IE 6/7 began to download the files correctly. Apparently this IE XML bug:
pertains to any file downloads over HTTPS.