I only "speak" in my head when I'm reading a book. Then I have a narrator stuck in there, with different actors playing the roles of the people in the story. The narrator also reads poetry to me. Its voice sounds more melodious than mine.
A lot of the time, the "voice" in my head is singing to me. Sometimes, it is joined by an alto, tenor, and bass (I'm a soprano so the voice in my head is also a soprano), and I hear (in my head) four-part harmony. Usually, it's unaccompanied, but sometimes, I hear instruments. Then I get the urge to sing out loud.
Occasionally, I will perceive instrumental music in my head.
I am an auditory thinker but not really a verbal thinker.