Even if I use ?dl=1 I get the same error:

$ pandoc -s pandoc-test.md -o pandoc-test.epub
Fetching https://www.dropbox.com/s/xnrx1yrkzczvwk4/bach_toccata.mp3?dl=1...
pandoc: Could not find media `https://www.dropbox.com/s/xnrx1yrkzczvwk4/bach_toccata.mp3?dl=1', skipping...

Perhaps it depends on the version of pandoc?

I'm using the package in fedora 22:

$ pandoc --version
pandoc 1.13.2

I'll try the latest release (1.15.0.6) and let you know