Banquo speaks these lines in Shakespeare's Macbeth in act 1, scene 3. Let's look at the context of these lines. Banquo and Macbeth enter the scene after a fierce battle in which Macbeth fought heroically and nearly single-handedly won the day:

MACBETH: So foul and fair a day I have not seen. (1.3.39)

This is the first line that Macbeth speaks in this scene and in the play. This line sums up the whole play quite well for Macbeth, as Shakespeare no doubt intended. What happens to him throughout the play can be seen as both foul and fair.

Macbeth's first line also echoes the Witches' closing lines in the first scene of the play:

WITCHES: Fair is foul, and foul is fair.
Hover through the fog and filthy air. (1.1.11–12)

(Closing a scene with a rhyming couplet is a device that Shakespeare uses frequently in his plays. He does it seventeen times in Macbeth.)

Then Macbeth and Banquo meet the Witches:

FIRST WITCH: All hail, Macbeth! hail to thee, Thane of Glamis!

SECOND WITCH: All hail, Macbeth! hail to thee, Thane of
Cawdor!

THIRD WITCH: All hail, Macbeth, that shalt be King hereafter! (1.3.50–53)

That's when Banquo says:

BANQUO: Good sir, why do you start, and seem to fear
Things that do sound so fair? (1.3.54–55)

Macbeth is already Thane of Glamis, so that shouldn't surprise him. He knows that the Thane of Cawdor is still alive (but not for long, although Macbeth doesn't know that), so he doesn't take that prediction particularly seriously, and he doesn't react to it.

But why does Macbeth "start" and "seem to fear" being told that he "shalt be King hereafter"? It's a startling thing to hear, certainly, but Macbeth doesn't seem to be the kind of man who is easily startled or who fears much of anything. Banquo might also mean that it's "fair" that Macbeth should be King, since he's earned it on the battlefield.

Why is Macbeth so taken aback by this particular prophecy? Was being King something that Macbeth was already thinking about? He seems to warm up to the idea of being King pretty quickly. By the end of the scene he's seriously thinking about it, even though the idea still frightens him:

MACBETH: why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? (1.3.145–148)

Within a few scenes, however, Macbeth has overcome his fear, and Lady Macbeth has convinced him to kill King Duncan and take the throne.

Literary devices that Shakespeare uses in Banquo's lines include alliteration and antithesis, and if we stretch the devices to include more of the scene, they include anaphora and repetition.

Alliteration is the use of words or phrases that start with the same sound, such as "start" and "same sound" and "such" (in this sentence) as well as "sir . . . start . . .seem," "sound so," and "fear . . . fair" (in Macbeth). Antithesis is the contrast between two opposing ideas, such as "fear" and "fair" and the contrast of "do you start," "seem to fear," and "sound so fair."

Anaphora is repeatedly using words or phrases at the beginning of a number of sentences (or lines), such as "Hail, Macbeth! Hail to thee." Furthermore, repetition is exemplified in the scene by "foul . . . fair" and "fear . . . fair," which also repeat the same words used by the Witches earlier in the play. It is also used by Shakespeare to emphasize the "foul . . . fair" theme in this scene and in the play as a whole.

In simple terms, what is being asked of Macbeth here is: "Why are you so jumpy, as if you're afraid of things that actually sound great?"

Of course, Shakespeare's phrasing is much more elegant! The literary and rhetorical device he is using to create effect in this quotation is antithesis. Antithesis is a device whereby two contrasting ideas are set together in a parallel sentence structure, which we also see elsewhere in the play from the Witches—"Fair is foul, and foul is fair." Here, the phrases "seem to fear" and "sound so fair" parallel each other, but the "fear" contrasts with the "fair" to underline the strangeness of Macbeth's reaction, while also echoing the Witches' earlier lines. This helps draw the audience into the play, as it reminds them of the full context.

Meanwhile, devices such as assonance ("seem to fear") also help to give this phrase a poetic, chant-like quality, again similar to the Witches' chant.

The literary device used is alliteration (repetition of initial consanant sounds) with the repeated "s" sounds: sir, start, seem, sound, so. The next step is to consider what symbolic meaning (if any) could be conveyed, and in this case, one would think of a snake, or serpent, in that they make that sound. This fits with the context of the play, in that there is evil all around as the weird sisters give their prophecies and the seed of Macbeth's demise is planted.