This video offers a concise description of the development of the Greek language and certain Greek vowel sounds.

“We may not know exactly how the Athenians of the classical period sounded, but we do know that Neohellenic, the direct descendant of Attic Greek, preserves the Eucleidian alphabet, orthography, and historical Greek sounds. The same is true of the Greek of the New Testament.”

By Philemon Zachariou.

https://www.youtube.com/watch?v=LAxzL1BE3Go