There's some idea of ancient greek pronunciation based on phonetic misspellings found scattered about. I studied /some/ ancient greek in undergrad, and when I visited Greece it did, indeed, seen like a while different language, with large shifts in both pronunciation and meaning of words. My favorite was agathos, which is noble and good in ancient greek, but means native or simple minded in modern greek.

Also worth keeping in mind that broadcast and recordings do a lot of work to reduce language drift today, over time but especially over geography. A mountainous, watery place like Greece almost certainly would have a lot of drift in accent and informal language.

[On edit: That feeling when you swipe out a long comment, hit submit, and see a fresh neighboring long comment making the same points better.]