How does Kakfa develop this relationship between voice and power? (Or lack of both?)

I would look at this more from the perspective that loss of power (meaning Gregor can no longer take care of his family), leads to a loss of voice because he simply wants to disappear. The past doesn't matter, and although it's a struggle, his family slowly steps up to the plate and becomes independent of what Gregor can no longer provide. He then becomes a nuisance, who eventually isn't noticed at all.