In japanese, it was エアリス literally EARISU.Think it pronounced Ariel when it was spell Aerith, that's why I was going with that.
because I can't really get how you can get an "s" sound out of "-th"...But then it could be a strange Japanese things.......I have learn from a very early ages not to question Japanese way.......Otherwise most of their game make no sense......
The closest is Aeris, Aerith sounds like you have to twist your tongue and spout out some waters.
EDIT: I got it wrong, the official English name is now Aerith. Aeris was the version in PSX, man, they choose the mouthful one.