When did the pronunciation of words change????

I'm listening to several podcasts with millennial and young contributors and can't help but notice that the pronunciation of common words have changed (well at least from how I was taught to say them). For example, mountain. When did it become mount-in? Or button, now butt-in. My least favorite of the bunch? Impor-ent. It's everywhere! It's driving me batty! Or should I say bat-ee lol.