Final answer:
Lady Macbeth hears the cry of an owl while Macbeth is killing the king, which is symbolic of doom and the evil deed occurring.
Step-by-step explanation:
The cry that Lady Macbeth hears while Macbeth is killing the king is that of an owl. This is made evident in Act 2, Scene 2 of Shakespeare's play, where Lady Macbeth says, "It was the owl that shriek'd, the fatal bellman, Which gives the stern'st good-night." The owl's cry is symbolic, adding to the dark and ominous atmosphere of the play, representing doom and acting as a portent for the evil deed that is about to transpire.