My sister was like this growing up. Like GeorgeC, she is a musician, but growing up it was mostly chatter and talking, IIRC. And yes, she did it even in her sleep. As the very quiet, introverted sibling with some noise sensitivity, it drove me crazy, and is something that still colors my memories of our sibling relationship. I used to believe it was a ploy for attention- she was insatiable in that regard (though obviously talking and humming in your sleep doesn't fit that theory).