I solve it using two soundcards and default winamp input.
But i think its possible to do the same with one card.
You need an output for the sound that you want to use with milkdrop. Put the output in an input that is your default recording device which is used to feed "winamp input" and milkdrop. I did that using one cable from one sound card to another, but i think its possible with 1 card. Try doing the same thing with a cable and switch sound monitoring off.
I hope it helps.