"Not marble nor the gilded monuments"
The repetition of the "m" sound in "marble" and "monuments" creates an alliterative effect.
"Of princes shall outlive this powerful rhyme"
The repetition of the "p" sound in "princes" and "powerful" creates an alliterative effect.
"But you shall shine more bright in these contents"
The repetition of the "b" sound in "bright" and "these" creates an alliterative effect.