No idea how they were pronounced when they were fresh and new ... but in English we'd usually say EYE-sis unless we were singing Mozart, in which case we'd use the continental version: EE-sis.
Similarly, in English we'd say eye-ZI-a (that's the same vowel sound in the first two syllables, folks - wish this keyboard could do the IPA!) unless we were acting in Guys and Dolls, where it seems to be eye-ZEY-a with the stressed syllable rhyming with way and hey. I don't know if this is widespread in the USA!
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