This entire conversation is new and fascinating to me. I used to be much more visual but I would say that my primary mode of thinking now is monologic. However I would never think to describe it as hearing. It is more like an imaginary 'voice' that is not the one I use with other people (it is absent timbre etc and just a much simpler more efficient voice abstracted from physical constraints). I use it to write out my thoughts in my mind. I quasi-visualize the chain of words just in a very abstract, largely amorphous way using this 'voice' and play them through my mind while 'speaking'/'hearing' them. In this sense I pretty literally get 'lost in my train of thought' when thinking about something interesting.

I don't know how much this style of thought came about because I really didn't enjoy constantly visualizing things and consistently fought against it; mainly due to the constant intrusive visual sequences of various types, of which I still have far too many for my taste. I use schematic and visual imaginations all the time, but I guess I feel safer using the monologue and so it has become the bulk of my thought. At the same time the fact that I have learned several languages and have had to consciously focus on language over the years undoubtedly plays a big part. I wonder how many polyglots who learned languages later in life use a similar monologic imagination style?