Doesn't really matter what method they used, the fact they got a vocalised song is enough to show that the SNES is capable enough for doing the same during the Opera scene (although I doubt they'd worked out how back in FF6's time) - all the Opera scene would require is several smaller pieces of song which would play back-to-back, the selection of which pieces play depending on whether you choose the right lyrics during the game or not. The same sound engine could just be used though.

As for voices in battle, if by simple and repetitive you mean all the characters have their own voices and call out the name of the spell/technique they are performing, then yeah, that's what they do Tales of Phantasia on the SNES has over 100 of these voice samples. Star Ocean has more since it also has phrases for each character for different situations and more characters.

It's a shame these two games weren't officially translated so you'll never find them in your usual English/American game stores, but you can always download ROM images of them off the internet and run them through an emulator to see the technology they use