Aeris or Aerieth

[Squall of SeeD]

And in Latin, "th" is pronounced "s." You guys ever listened to One Winged Angel? The choir pronouces it "Sephiross." But....it's a personal preference of mine to say Sephiroth (pronounced as spelled).

"Aerith" isn't Latin and "th" is not pronounced with an "s" sound in Latin ("Aeris" is a Latin word, but considering the Katakana of Aerith's name is "Earisu," taking into account Japanese phoenetic values ("th" having an "su" sound), that "Erith" is the Hebrew word for "Flower," and Aerith's connection to flowers, "Erith" is much more likely the origin of her name). "Th" carries a sound similar to the "s" sound in Japanese ("su"), which is what you're likely confusing this with.

As for Sephiroth's name, the Japanese translations would have it as "Sephiros" or "Sefirosu," so the choir saying it as such is understandable. The name comes from the Kaballah of Jewish Mysticism, in which it is pronounced "Seh-fer-oth." I hope this helps.