Phonology
The pronunciation of Post-Classic Greek changed considerably from Ancient Greek, although the orthography still reflects features of the older language (see W. Sidney Allen, Vox Graeca – a guide to the pronunciation of Classical Greek). For a detailed description on the phonology changes from Ancient to Hellenistic periods of the Greek language, see the article on Koine Greek.
The examples below are intended to represent Attic Greek in the 5th century BC. Although ancient pronunciation can never be reconstructed with certainty, Greek in particular is very well documented from this period, and there is little disagreement among linguists as to the general nature of the sounds that the letters represented.
Read more about this topic: Ancient Greek