Absolutely my son has always done this and still does, I always hear him chatting away to himself.in his bedroom, No one has mentioned it at school but they seen to know and accept him, and I'm sure he does talk himself through things.
I posted this problem amongst all the others, a good few years ago on another site here:- http://www.beingdyslexic.co.uk/forums/index.php?showtopic=1579&st=0&p=8522&hl="internal%20dialogue"&fromsearch=1&#entry8522 if you go tp post #20 there seems a good explanation of what could be happening, although Matthew can read quietly these days..
I mysel,f am often caught "talking to myself", colleagues at work often comment that my mouth is moving, I know I do it, my thoughts just seem to overflow, I don't think it's something to worry about - well I hope not!!