Tifa ~ Tiff-ah
...
Just kidding

Tee-fah
Aerith ~ Ayr-ith (I find pronouncing the "s" in Aeris too harsh, so I prefer the softer "th")
Ifrit ~ Iff-ritt (Not If-reet, like Balthier)
Vivi ~ Viv-ee (Not Vee-Vee, like KH2)
Ramuh ~ Ra-moo
Eiko ~ Ee-ko
Quina ~ Qwin-ah
Seifer ~ Sigh-fer
Quistits ~ (OK, Quistis) ~ Qwis-tis