That mental voice thing sounds pretty much what I do, too. I twirl and twist ideas and sentences in my head, and when something interesting appears I need to write it down quickly before it runs away. Writing or typing doesn't interfere with it much if I'm really quick about it, but trying to say it out loud blows it all away. (Especially when I'm thinking in English, which I often am, since speaking it is always a bit of a challenge. I have no idea how much of this applies to writing in Finnish, because I haven't done that in ages despite it being my mother tongue.)