“Instruments of darkness” (Bangui Act 1)
Auditory image on “instruments” the auditory imagery implies a malevolent musicality the words possess an almost hypnotic trance for Macbeth - their speech is a manipulative melodic composition
Plosive on “darkness” creates a harsh sound foreshadowing the ominous and grim destiny the witches are ready to inflict upon Macbeth
Banquos discernment (able to judge it well) of this “darkness” indicates his immunity to the hypnotic and malevolent musicality that captivates Macbeth