The interpretation of "Have you anything to say in your defense?" by Cesar Vallejo?

I just read this poem today and I don't really understand it. Especially when he says "There is an empty place in my metaphysical shape that no one can reach: a cloister of silence that spoke with the fire of its voice muffled". What are the empty place, metaphysical shape, a cloister of silence represent for? and why is poet saying that the cloister of silence that spoke with the fires of its voice muffled?