Ancient Greeks wrote in capital letters without accents or punctuation, so we cannot really know how exactly they pronounced things. During the
Alexandrine era of globalization, when Greek became what English has become today (a lingua franca) a set of accents and breathing marks were devised so as to help out folks around the world to read and pronounce Greek “properly”.
So for at least the
Alexandrine era on, we know how words and names are accented. All records on the name from the time
Medea include an accent on the first syllable. Of course, accents and dialects differed back then (I am sue Dorians made fun of Ionians and vv) as they differ now. One can tell an Athenian apart from, say, a Cupriot, pretty much as easily as one can tell apart
Bill Clinton from
Tony Blair. So don’t believe anyone who claims to know how exactly ancient Greek was pronounced. There are a few differences that are however clear: vowels with a rough breathing are believed to have had a hissing sound back then which is completely lost today in Greece -- but interestingly enough somehow made it in English via Latin. Consider the prefix "Hyper-". In Greek it is written "Yper-" with a rough breathing. Contemporary Greeks do not pronounce the H, but it ancient Greeks most definitely did. Of maybe is sounded like an S, as such words the word has also been transliterated with a S (“Hyper” and "Super-" are both from the Greek YPER).
Ah, the fun we have :)