Well you can have a good sample if you listen to ecclesiastic language. Its basically attic koine. language changes nobody said no, but it doesnt change to a point beyond recognition if its spoken all the time. it just evolves.

As for dipthongs. An example:αι: as in Cairo or high
ai is not ''a-i'' its ''ai'' as in''chemical'' dipthongs are 2 letters pronounced as one sound not 2.