Has pronunciation of words always been the same?

The way we say certain words, how do we know they are supposed to be pronounced the way we pronounce them? What if they were said differently when they were created? Also I believe this could also apply to the names of historical rulers.

For example Genghis Khan, we say it very casually, example (gehn giss kaan) I think it may have been pronounced with as much gusto as humanly possible, the potential reason being that if someone were to say his name lazily they might have been murdered, I also believe that by extension in modern day, us not doing this may contribute to the normalisation of acceptance of these otherwise disgusting deplorable genocidal bastards, even Hitler is getting this same treatment, the pronouncement of his name becoming more and more casual, I feel bastards like those their names should be said as you would the name of satan, although even satans name is said too casually.

Back to my first point, names we have become accustomed to could have been said totally different years ago, I'm not sure if there are records that exist on the correct way to say these words, not just the names of people but things (examples: space, the sun, the moon, stars, gravity) but I feel as though this could reflect on society's shifting priorities/interests.