At the time, did you have to load the soundfont manually to use it with FF7?
If that's the case, then that's your explaination for the lack of voices.
You see, under ideal driver conditions (which might be now) when you launch FF7, it interfaces with the SoundFont Manager system (without this, you can't load soundfonts AT ALL, so it would have definitely been installed), to auto-load the correct size soundfont for FF7. This size-checking step only exists because back then, a majority of Sound Blaster cards were of the AWE model, which used onboard "SF-RAM" (a term I made up, naturally, it means "SoundFont RAM") to store Soundfonts instead of using your System RAM, and at max you might have 4 or 8MB of the stuff availible to your card, which is why you see that they have FF7's Soundfont in 512k, 2MB, and 4MB sizes....
...ANYWAY, in addition to the different-sized soundfonts, there was another font that is ONLY auto-loaded by the game into SF-RAM when it is playing the MIDI version of One Winged Angel, and then removed from SF-RAM after the fight is over.
Now, the problem that comes up here, is that since the SBLive! family of cards had a slightly different way of loading up soundfonts, tended to result in FF7 not able to correctly interface with the SF Manager and thus couldn't load the soundfonts by itself. This may also be the reason why the config program tends to give you a "Failed!" result when you try to test the soundfont loading.
Now, I'm doing all this off the top of my head, I think I have most of the history concerned with this bug correct, but I'm not 100% confident this is how it is exactly.....Rubicant, you have a Live!/Audigy, right? Mind telling me if any of this is wrong?