I’m not sure that’s accurate. When I read and subvocalize I do have the sensation of “hearing” the voice in my head. Same for thinking. It’s more like remembering what someone said though since there isn’t an actual external source. For example I can read a sentence in Sean Connery’s voice but it’s more like remembering what he sounds like but transferred to the words I’m reading. My own inner voice does have a unique sound to it too, not quite the same as my speaking voice though.