After committing the murder, Macbeth says,
Methought I heard a voice cry, "Sleep no more!Macbeth does murder sleep"—the innocent sleep,Sleep that knits up the raveled sleave of care,The death of each day’s life, sore labor’s bath,Balm of hurt minds, great nature’s second course,Chief nourisher in life’s feast. (2.2.35-40)
In addition to showing us the level of guilt that Macbeth feels for killing a good and kind king such as Duncan, this hallucination of Macbeth's indicates his awareness that he hasn't just killed a king (and friend and cousin), but he has actually committed a crime against Nature. In one fell swoop, Macbeth has killed his guest, his kinsman, and his king, and he has begun a war against the natural order. He recognizes how dastardly it was to kill the poor man while he slept, defenseless: sleep is supposed to help ease our cares, soothe our exhausted bodies and minds, and give us energy to begin again tomorrow. Instead, Duncan's sleep rendered him defenseless and Macbeth has committed a crime against country (Duncan is king), God (kings are ordained by God), and Nature.
In Shakespeare's Macbeth, Macbeth hears a voice
...cry "Sleep no more!
Macbeth does murder sleep,..." (Act 2.2.38-39)
Still it cried "Sleep no more!" to all the house;
"Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more! Macbeth shall sleep no more!" (Act 2.2.44-45)
Glamis and Cawdor both refer to Macbeth, of course.
Macbeth's hearing voices demonstrates the depth of the guilt he feels for assassinating a just, humble king like Duncan.
Before Macbeth murders Duncan, his guilt at the thought of doing so is demonstrated by his vision of the bloody dagger. Here, after the act, his guilt is demonstrated by the voice he hears, as well as, by the way, his inability to choke out an "Amen" when he tries to join in with a prayer he overhears after he stabs Duncan to death.
In Act II, Scene 2, we see Macbeth talking about what happened after he killed Duncan. What he thinks happened really shows how guilty he feels about what he has done.
After he killed Duncan, he imagined he heard someone calling him a murderer. No one was really actually saying this, but he thinks he heard it. That shows that he feels guilty -- he is accusing himself of what he has done.
Macbeth also finds himself unable to pray. Both of these things show that he really feels that he has done something very wrong.