Reading Sonnet 55 is like sitting back in a fat armchair and tuning into some afternoon Bach on your local classical radio station. There's nothing warm and fuzzy about what's actually going down in the poem, but thanks to the stately gallop of iambic pentameter (daDUM daDUM daDUM) and the clever symmetry of all that alliteration, assonance, and internal rhymes, our ears hear nothing but elegance and balance.
Almost every line is decorated with a little alliteration (one or more words that begin with the same sound), and some go absolutely hog-wild. Take line 5, for example, that goes all out with the W's and the S's:
When wasteful war shall statues overturn (5)
But notice that Shakes keeps the line totally under control. There's a 3-sound pattern here with three W sounds ("when," "wasteful," "war"), immediately followed by three S sounds ("shall," "statues"). Alliteration usually refers to the sounds that begin words, but because "statues" ends with an S and comes right before a word that begins with a vowel ("overturn"), the S and O run together like a new word.
Plus, Shakespeare doesn't confine his sound effects to the front ends only. Get a load of line 3, whose bonanza of S and T sounds duck in and out of words like the slippery mold they're describing:
Than unswept stone besmear'd with sluttish time (4)
Shakespeare keeps the sonic harmonies dancing with an equally clever use of assonance: the repetition of vowel sounds inside words. Like the alliteration, these guys are all over the place, abundant but balanced. At the beginning of the poem, for example, we've got four short I words that spill across two lines:
Not marble, nor the gilded monuments
Of princes, shall outlive this powerful rhyme (1-2)
Since this sentence also wraps around two lines, the scattering of similar-sounding words is important to tying everything together. In the final two lines, we've got another balancing act, this time between short I and long I assonance:
So, till the judgment that yourself arise,
You live in this, and dwell in lovers' eyes (13-14)
Here a lot of short I sounds in short words (till, live, in, and this) mirror the two long I sounds at the end of the lines: arise and eyes. These ending rhymes naturally have more weight because of their position, but the contrast with the short I sounds reminds us that arising and dwelling in lovers' eyes is only possible if the beloved first lives in this (the poem).
But all this snappy back-and-forthing isn't worth much if your eyes get bored. Shakes keeps it lively with a sprinkling of visual rhymes, like the almost-rhyme "shall still" in line 10. But it's line 6 where the fun really gets out of hand:
And broils root out the work of masonry (6)
We've got 7 (count 'em) O's lined up like pigeons on a branch: broils, root, out, work, of, masonry. So, what's the joke? Well, the line's about war messing up stonework, and all those O's make the destruction visible: they've already knocked holes into the line itself. The white page shows through, just like the blue sky will, once the broils have their way with the masonry.