Many, many black folks in the south say "mmmm hmmm" instead of "You are Welcome".

Not only black folks and not only in south. I used to say that too, sugah. Wonder why I stopped? I think it happened in my 30s when I went to uni.

I was in the Southest US this past summer, it was wonderful feeling simply talking to fellow shoppers and shop assistants in the same accent. That welcoming friendliness of being home perhaps. I don't get same in like environment in UK. I get shoppers knocking me aside, trodding on my feet and running into my shins with push chairs and snarly unhelpful shop assistants.

Sigh.