What instances of alliteration and assonance are in Gonzalo’s speech? What effect does the use of alliteration and assonance have on the reader?

Gonzalo is making a speech about what kind of world we'd have if he were in charge. It's a utopian vision; there'd be no war, riches, poverty, or any kind of laws. Humankind would be innocent and pure. Gonzalo's utopia would've seemed as strange to Shakespeare's audience as it does to us today, so it's necessary for him to use rhetoric to put his point across. Rhetoric is the art of persuasion, and we'd certainly need persuading that a society without education, for example, would be one worth having.
Throughout his little speech, Gonzalo uses both alliteration and assonance. Alliteration is where you have a series of words close together with the same first consonant sound. Assonance takes place when vowel sounds are repeated in words close together.
Alliteration is an important rhetorical tool, as it draws attention to itself, making us focus more closely on what's being said. And as that's the whole purpose of Gonzalo's speech, we can see why Shakespeare uses it. An example:

"All men idle, all . . ."

This is an especially important point for Gonzalo to make. In his ideal commonwealth, men would no longer have to work to survive. They, like everyone else, would share equally in the bounteous wonders of mother nature.


"Contract, succession,

Bourn, bound of land . . ."



Alliteration isn't just used to give us an idea of what the brave new world would be like; it also reminds us of the world we'd be rejecting, a world of contract, inheritance, and property laws.
 
Assonance is used to set the tone of Gonzalo's rejection of the old world. The repetition of the "o" vowel sound slows down the speech, forcing us to sit up and take notice. It also has a suitably condemnatory tone to it, as if Gonzalo is some kind of Grand Inquisitor setting out a litany of woes afflicting society that will no longer exist in his shiny new paradise:


"For no kind of traffic
Would I admit. No name of magistrate . . .


No use of metal, corn, or wine, or oil.
No occupation . . . No sovereignty—"



Gonzalo is wearing us all down with his constant negativity. And the insistent drip feed of "o" sounds, at the very least, gives us pause for thought. We may not agree with Gonzalo, but Shakespeare's skillful use of these literary devices means that we are nonetheless compelled to listen.