As a reply to others who may have viewed this post, Firebrand was on the right track, and I thank him.
While I turned off the music outside of Vavoom with the WINXP MIDI mute, I didn't experiment with the volume for this slider and, in fact, it does work. The trick of it is to get the MIDI volume balanced with the in-game sound volume and then the overall sound balanced, as well. A mean trick, to be sure, but I think I am OK. Of course, if the in-game music volume worked, no gymnastics would be needed - but, there you go.