I never really used it, and I don't think that w2k and above allow it to be used.
Well, you can see that the option is available in my screen shot above, and I'm using Windows XP

Anyway, I don't think it works on the SBLive! synths, but its available because my Yamaha XG Synth supports it. However, if it is turned on, as in the screenshot, the volume slider does not function correctly, as Goku pointed out in the previous post (you have to move the thing way to the left before it starts to get quiet).
Ah well, I don't really know how to make the soundfonts work if they don't, cause I've been using the Yamaha Synth because it is the %$@# coolest thing ever. Only bad thing about it is you don't get the voices in the final battle music.