Shakespeare creates a tense atmosphere (or mood) in this scene beginning with Lady Macbeth's initial concern that Duncan's grooms have awoken and, in doing so, have prevented Macbeth from being able to complete the murder. Then, the quick back-and-forth between the husband and wife when he enters the scene increases tension. The nervous questions and short, terse, replies contribute to this anxious atmosphere. Further, both of them have heard strange noises for which they cannot account, including the voice Macbeth heard that has promised that he will not be able to sleep peacefully anymore.
In addition, when Macbeth and Lady Macbeth begin to conflict with one another, this adds to the tense atmosphere too. He brought the murder weapons with him rather than leave them near Duncan's body and the men they are attempting to frame, and when he refuses to return them himself, Lady Macbeth scolds him for his lack of courage and returns them herself. Once she does so, the couple begin to hear a knocking at the gate: each time they hear a knock, the tension increases because they know that Duncan's body will soon be discovered and they will have to pretend innocence.