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Answer by Paul King, computational neuroscientist, data scientist, software entrepreneur:
There are many theories about this, but no definitive answers just yet.
There is the phenomenon of “inner speech,” which refers to the sense that you can “hear” yourself thinking. This is that “silent voice” that narrates your day-to-day activities. When you imagine what you might say to someone and his or her response, you are “hearing” a voice of sorts, but you know it is not an actual sound. This type of auditory imagination is a bit like visual imagery, where you can imagine what something might look like (like imagining a frog and zooming in on one of its eyes) without actually seeing it.
Another phenomenon mentioned in other answers is subvocalization—the act of reading actually activates the muscles in the throat, vocal cords, and sometimes the lips. When we learn to speak, we learn to make sounds with our mouths. Learning to read also often involves reading out loud, in which case we hear our own voice. As vocalization is suppressed in order to read silently, the muscles may still move slightly, and we “hear” what we know we would sound like if we were to speak out loud.
There is a nerve bundle in the brain called the arcuate fasciculus that connects the speaking part of the brain (Broca's area) to the speech understanding part (Wernicke's area). This nerve bundle seems to play a role in the experience of inner speech.
Incidentally, inner speech does show up in fMRI brain scans as activation of the speech areas of the brain, so whatever is going on does have an identifiable neural correlate.
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