The voice that you hear whenever you sight-read (read in your head) is called:
internal speech, inner speech, inner voice, and more specifically, the inner reading voice. If the inner voice heard is different from the reader's normal voice, it is said to be the writer's/author's voice (aka the narrator's voice)
AS YOU begin to read this article and your eyes follow the words across the page, you may be aware of a voice in your head silently muttering along. Most of us do it when we talk, listen, write letters or emails. The very same thing is happening to me as I type these words: a private internal narrative is shaping the words before I commit them to text.
What if we could tap into another person’s inner voice? Thinking of words does, after all, create characteristic electrical signals in our brains, and decoding them could make it possible to piece together our thoughts. Such an ability would have phenomenal prospects, not least for people unable to communicate as a result of brain damage.
From Research Digest
Among the contributors with an internal reading voice, another key theme was whether or not they only ever heard the same voice (this was true for about half of them) or a range of different voices. For those who heard different inner voices, these tended to vary based on the voice of the character who was speaking in a story, or if it was a text message or email, or the voice of the sender. For people who only ever heard the same internal reading voice, this was usually their own voice, but it was often different in some way from their speaking voice, for example in terms of pitch or emotional tone. Some contributors described or implied that their inner reading voice was just the same as the inner voice they used for thoughts.
If the reader strongly dislikes the inner voice they hear, the person could be suffering from auditory verbal hallucinations (AVH)
[…] Some contributors spoke of the voice as distracting or even scary, while others said they deliberately chose the voice they used. You can see why this paper was published in the journal Psychosis. Indeed, Vilhauer said that the insights from her analysis provided some support for theories that say auditory hallucinations are inner voices that are incorrectly identified as not belonging to the self.