Try adding mp3 content type header:
It is likely that some of your users' IE browser would default to a document type unless one is explicitly specified.
Update: didn't realize you half answered your own question there, heh!
"We've all heard that a million monkeys banging on a million typewriters will eventually reproduce
the entire works of Shakespeare. Now, thanks to the Internet, we know this is not true."
Robert Wilensky, University of California
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