The Doctor's speech about "Foul whisperings are abroad ..." refers to Lady Macbeth's sleepwalking fits in Act V. He explains:

Unnatural deeds(65)
Do breed unnatural troubles: infected minds
To their deaf pillows will discharge their secrets:
More needs she the divine than the physician.

Through his observations, the Doctor has realized that Lady Macbeth is sleepwalking because of a guilty conscience. She refers to "blood" and "the old man" indicating to the Doctor that she feels extremely guilty for the murder of King Duncan. The Doctor recognizes the murder as "foul whisperings." This contributes to the theme of "fair is foul and foul is fair" because it shows that Lady Macbeth's fair return (just return) for encouraging murder is a dark conscience.

Later, at the end of Act V, scene v, Macbeth recognizes that Birnam Wood is heading toward Dunsinane Hill. This fits with the theme of "fair is foul and foul is fair" because Macbeth felt assured of his safety when the third witch told him, "Macbeth never harmed be until great Birnam Wood come to Dunsinane." Macbeth had become so full of hubris (arrogant pride) that he was unable to discern deeper meaning than the words. He only understood the question, "Who can enchant the forest, bid the trees to unfix their root," which implies that no one has the power to actually make the forest move. This, of course, is true, but his excessive pride prevented him thinking like the military he formerly was: a war tactic is to chop down the trees and use them as a means of protection--which is exactly what Siward and the English army did to overthrow Macbeth. This episode also fits with "fair is foul and foul is fair" because the one assurance that Macbeth thought he had, that of the forest not moving, was exactly the thing that brought him down.