Macbeth says he heard many sounds, mostly voices. He heard someone in his sleep cry out, "God bless us!" and another cry out, "Amen!". When Macbeth himself wanted to say, "Amen" in return, the words stuck in his throat. The significance is that he has just killed a king and he has broken his connection to God and so cannot say the word. Macbeth also says he thought he heard someone say, "Sleep no more! Macbeth does murder sleep". The significance of this is that Macbeth suffers from insomnia ever after this. Macbeth also heard, "Glamis hath murdered sleep, and therefore Cawdor / Shall sleep no more! Macbeth shall sleep no more!" Macbeth has come into Lady Macbeth's chamber holding the bloody daggers and with blood on his hands. He is in a state of shock and seems remorseful. Lady Macbeth, who appears much calmer and cooler, chides him for forgetting to leave the bloody daggers on the guards and takes the daggers to place them on the guards in order to implicate them in the murder of Duncan. She tells him she'd be ashamed to be as remorseful as he is and that a little water cleanses them both literally (as it washes off the blood) and figuratively. In Act 5, she will try to wash blood from her hands. Macbeth's reaction to the death of his wife is very different from what we, as an audience expect from a man who shared a very intimate and close understanding with his better half. Macbeth, early in the play, derived trememdous insipration from, and was heavily influenced by his wife, Lady Macbeth, who then, seemed to be one of the most ruthless, power-hungry female characters created by Shakespeare. When Macbeth finds out Lady Macbeth comitted suicide, he is emotionless, saying "she should have died hereafter" ( see act 5, scene 5).
Macbeth is very calm, almost like he doesn't care at all. He says that she was going to die someday anyway. He has lost everything and this does not bother him.
On the other hand you might view the reaction of Macbeth to his wife's death, as one of neutral remorse. He realized that he should be saddened by the event, but was too busy trying to protect himself.
The brave, noble general whose children, according to the witches' prophecy, will inherit the Scottish throne. Like Macbeth, Banquo thinks ambitious thoughts, but he does not translate those thoughts into action. In a sense, Banquo's character stands as a rebuke to Macbeth, since he represents the path Macbeth chose not to take: a path in which ambition need not lead to betrayal and murder. Appropriately, then, it is Banquo's ghost—and not Duncan's—that haunts Macbeth. In addition to embodying Macbeth's guilt for killing Banquo, the ghost also reminds Macbeth that he did not emulate Banquo's reaction to the witches' prophecy.