That is a incredible claim. Is common that all the languages across history to have an explanation in how was pronounced or have sound samples of how the people talk about?
Because that is the only sure way to do that claim.
Just taking in account the changes I see in my own city, is impossible that words not change the sound. Heck, you can bet it change among people in the same HOUSE!
ie: I truly want to know how this can be proved...