I think it's just me being shocked that someone would mess with the voice I always liked so much. Hearing such a different voice coming out of her in the scene throws it all off for me and changes my opinion from a "hey, Celes actually has a nice singing voice" to "oh god why, my ears, noooo"
(Yes, I am saying I prefer the GBA synth over the Italian voice, that's my personal opinion. Hell, I like the original SNES over the voice. But as I said, there's a volume switch for a reason, so feel free to ignore my complaints.)