I'm having an interesting issue with our .m4a (AAC) audio sample files after upgrading from 1.2.6d to 1.3.7.

For some reason, customers running Windows (we think we've narrowed it to XP), who have shopped with us before and successfully listened to the samples on our site, are no longer able to just click and listen. The link appears to them to be loading, but they get a blank page with that pesky little "missing file" red X. They can right click and "Save target as. . .", then open the file with iTunes, but, before the upgrade, the files would load and play successfully with iTunes, RealPlayer, or WinAmp with a single click.

Does anyone have any idea why this would happen after upgrading?

Here's a link to the site