The most fitting interpretation of the owl that Lady Macbeth claims to hear is:
Lady Macbeth is beginning to lose her sanity and only thinks she hears an owl shrieking.
In this context, the owl serves as a symbol of death and foreboding, representing the ominous atmosphere surrounding the murder they are about to commit. Her mention of the owl highlights her anxiety and guilt, suggesting that the weight of their actions is starting to affect her mental state.