This seems fairly obvious, we don't use English to communicate to ourselves to move our fingers over the keyboard for example, or breath, or do any movements. Even visualizing stuff in the 'minds eye' doesn't require language, you just 'do' it. It's not much of a leap to think that the English we hear in our head comes from somewhere too, a more basal/instinctual level. If you look up speed reading too, you'll find that the people try to unlearn the hearing of the voice in their head when they see a word, in order to be able to process more words quicker.
As for the 'tulpa' link, that might be one of the stranger thing I have seen in a while! I think it is natural to hear other peoples voices in your head that you know from real life, when you you hear their reactions to some action you do. But I've never heard of training a whole new voice from scratch! Anythings possible I guess.
If you find it fascinating/strange you might be interested in this snazzy article (which is the one which linked me to the concept to begin with) that discusses plausible (but speculative) mechanisms for both this and other mental phenomena, including dissociative identity disorder and certain religious experiences: http://www.meltingasphalt.com/neurons-gone-wild/
(I think I came across the link in the comment pages here not too long ago, and it's been posted to the main news stream before, a while ago.)
Not to mention programming. Tracing/composing an algorithm or larger program is entirely visual and semantic, like flowcharts. There's no need for internal monologue using spoken language.
I can understand getting in someones head, and trying to think as they do. But hearing their voice, and seeing them, and talking to them and they reply like a separate person? I'm not so sure about that..
As for the 'tulpa' link, that might be one of the stranger thing I have seen in a while! I think it is natural to hear other peoples voices in your head that you know from real life, when you you hear their reactions to some action you do. But I've never heard of training a whole new voice from scratch! Anythings possible I guess.