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.
This was what Banquo said to Macbeth, on observing his startled reaction to the Witches' prophecies that were made on the deserted heath, after the 'hurlyburly' was done, the battle was won.
When the hurlyburly's done,
When the battle's lost and won. 1.1
The Witches' have prophesied Macbeth to become the Thane of Cowdor and the 'King hereafter': those were two promotions at the same time! These fortunate and good promotions were what Banquo said sounded 'so fair'. '[W]hy do you start; and seem to fear' is a reference to his worried, anxiety-ridden expression, his thoughtful face, which their words have given rise to because Macbeth's ambitious nature has always dreamt of becoming the King of Scotland, but having the agents of devil tell him that it was an actual possibility, was what shocked him beyond reality.
Banquo: The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence. 1.3