The best (and I believe, only) way to do this without multiple compilations out of WMM is to actually copy the music file, and then cut the end off the first and the start of the second (i.e. split it into two). Then lower the volume of the second piece (right-click, you can use a slider under "Volume".).
If you want a volume drop in the middle of a music clip, but then to go back up, you'll have to copy it three times and cut out the start, middle, and end as separates.