Dropbox, a popular file hosting site, has undergone some changes that resulted in some old download links now being broken. An explanation can be found here. They've dropped support for "public folders," which they'd been phasing out for awhile now. If you made your account after October, 2012, you should be fine.
I'd suggest that anyone who uses dropbox to host enhanced music or other files double check to make sure your links are working, and update your submissions if needed.