Macduff: "O horror, horror,horror, Tonge nor heart cannot conceive, nor name thee." (II. iii. 56-57) Macbeth and Lennox: "Whats the matter?" (II.iii. 58)
Macduff: Approach the chamber and destroy your sight...See and speak then yourselves. (II.iii.65-68)
Macbeth: There is nothing serious in mortality... And the mere less is left in this vault to brag of. (II.iii.86-89)
Macduff could be saying it very timidly with the extra pauses. Macbeth should be very surprised so they don't expect it was him. Macduff also should be shaking after what he saw.
Macduff should still be shaking at this point. Lennox and Macbeth should still be questioning what is happening. Macduff is in disbelief and at a loss for words.
Macbeth needs to sound like his is in remorse after finding out that king Duncan. He needs to emphasize how great of a man Duncan was. Make this very dramatic and maybe add a few tears or make it sound like you want revenge for whoever did this.