Milkdrop reacts to variation in volume, so as long as your "music" is not a flat tone of constant volume, there should be some reaction.
Check if your sound arrives in milkdrop at all. I myself had lots of problems with getting stereomix to work under windows 10 and f****** realtek drivers which have become so plug&play-smarty-farty that you completely lose overview what is going on when you changed one of the zillion options.
I also remember a bug in winamp which caused exactly this issue, no beat reaction on music. Search this forum for more information.
You may also consider to install the winamp community update version instead. It fixes a few bugs and is still under active development:
https://getwacup.com/