I never do it with open mouth. When someone thinks aloud I sometimes consider it normal: For example someone is solving a problem, while others are watching him solving it, and at a certain moment switches to saying aloud the steps he takes to overcome distraction. At other moments I consider it remarkable and wonder if the person behaving that way is maybe very lonely. In the beginning someone at my sport club was occasionally talking aloud. I wasn’t certain if he was talking to me or only thinking aloud and I asked what he was saying. Then I received the answer that he was thinking or determining how late he had to leave or so. At other moments he was talking to others much too softly.
Maybe how normal it is depends on country and era. I read somewhere that people started reading silently in the middle ages: A History of Reading - Steven Roger Fischer - Google Boeken