The vast majority of people have an ongoing conversation with themselves, an inner voice, that plays an important role in their daily lives. But between 5-10 per cent of the population do not have the same experience of an inner voice, and they find it more difficult to perform certain verbal memory...
I think I have an inner voice, but that's clearly not a voice I hear at all, that's the speech part not resulting in mouthing anything.
It's mostly true for foreign languages (like english) but less so for my native french.