FF7 PC Version had the soundfont files on the disc, one of them included the vocal bits to One Winged Angel in it. You can open them and use their sounds.
They're not in there as "slices" per se, there's a key you can slap that makes them yell "SEPHIROTH!" and other keys that will sing the phrase of "vetti vetti vetti whatever"
It's not like you have to program in the SE, then the PHI, then the ROTH, it's similar to how you'd record pieces in a DAW actually, little natural pieces