In Act I, Scene 5, the theme of Nature vs. Political Order is apparent in Lady Macbeth's observation that the raven who "croaks the fatal entrance of Duncan" becomes hoarse and cannot be heard. For, Lady Macbeth's unnatural political world, invoked with her calling upon the spirits to unsex her and fill her with "direst cruelty" that has no "compunctious visiting of nature," no natural feelings of pity, overtakes the natural world. Much like her husband who has called upon the predictions of the three witches, invoking the preternatural world to direct his destiny in his "vaulting ambition," Lady Macbeth assumes an unnatural state, as well, as she de-feminizes herself and embraces violence to further her political ends.
This soliloquy furthers the intentions to subvert nature to the design of the Macbeths. Lady Macbeth tries to use nature to hide her evil intentions as she calls upon the "thick night" and "blanket of the dark" so that her "keen knife see not the wound it makes." Their ambition for power thus leads the Macbeths to the phantasmagoric realm; that is, a shifting complex of things imagined and unnatural. This concept of unnaturalness in the Macbeths becomes their nemesis as in Elizabethan times, a common belief was that the health of the country was directly connected to the natural state, the goodness and "moral legitimacy" of the king.
"The raven himself is hoarse/ That croaks the fatal entrance of Duncan/ Under my battlements." If a raven was sitting on top of your house, squawking, it was a sign that someone was going to die, often of bubonic plague. Lady Macbeth's imaginary raven has croaked so hard, it's lost its voice. Remember also that the raven was one of the witch-familiar animals, so her choice of birds is appropriate. Notice the pronoun, "my battlements." She was talking before about psyching her husband up to do the deed. Now she has assumed control.