Tone in Macbeth
Shakespeare establishes a mysterious, chilling tone and conveys one of the main themes of the play: Things are not what they seem, and the witches suggest that perhaps what is good will be bad, and what is bad will be good.
The First Murderer's reply is likely spoken in a grim tone in order to address Macbeth's challenge of their bravery. This line indicates that they are not cowards and are strong and eager enough to seek revenge on Banquo.
Upon seeing the ghost again, Macbeth's tone changes drastically as he is roused by anger, yelling at the ghost to go away and leave him be. The effect of this violent tone of voice would have a significant impact on the guests in attendance.