We're undergoing one now

Staying with my 98 year-old mother, I listen to a lot of television while trying to read. Vowel sounds are changing, especially in the mouths of young women. I think that pushing a sound forward in a more closed mouth (obviously, I don't have a linguist's vocabulary) is the typical way that vowels shift.
So I hear, "We got this new peent on sell, and there's enough to due tyoo ryooms."
I won't be around 50 years from now. Just as well - nobody would be able to understand me.

You might want to consider who is making these programs and where they are supposed to be taking place. Even in the US there are many differences in pronunciation. British English can be quite different, and the regional differences are quite amazing. (There are parts of the UK that I won't make a telephone call to a company. In person it's not so bad.) The Australians are different again, not to forget the New Zealanders and the South Africans. Most Canadians are fairly easy for US Americans to understand. And that's just countries with large numbers of native speakers.

Thanks for a response, MJ. I'd say that generally these changes begin in California, get in the air or follow the sun, so that they are pretty much ubiquitous. The other noticeable thing is that young women are now pitching their voices very high so that they all sound younger than ten in my old ears. It's disconcerting to hear a scientist on NPR speaking about her research but sounding younger than adolescent.