This I find very interesting! There are a lot of people commenting that they do different voices for different characters. I personally dislike it when people do that (curiously, the few exceptions are some of the Discworld audio books, but even there I dislike it for most characters). I guess it may have something to do with it being a distraction when I feel like the voice doesn't sufficiently fit the character?
I also find it interesting that many people seem to consciously choose the voice in which they read things. I can change the voice if I make an effort, but when I'm focusing on the contents, the voice will tend to default back whichever voice I started out in. Making an effort to change the voice seems like it might take more energy than it's worth?