This poem's consonance—and especially its sibilance—help to evoke the speaker's complex feelings.
In the first half of the poem, the speaker's consonance suggests his relish of an imagined morning, a day when the sun seems to turn the landscape to gold. Listen to the gentle repeated consonant sounds in these lines:
Flatter the mountain-tops with sovereign eye,
Kissing with golden face the meadows green,
Gilding pale streams with heavenly alchemy;
Here, the speaker's delicate, tip-of-the-tongue /t/ sounds, whispery /s/ sounds, and muted /d/ and /g/ sounds all make it seem as if he's quietly relishing these words: this scene is as delicious to describe as it is to experience first-hand. And the long /l/ sounds of line 4 feel stretched-out and luxurious, just like those "streams" under the sun's golden touch.
But soon, the "basest clouds" come out and ruin everything. Describing this terrible change, the speaker uses some of the same sounds to very different effect:
Stealing unseen to west with this disgrace.
Now, those sibilant /s/ sounds feel more like an angry hiss or a stormy wind than a soft whisper. And the /t/ sound feels sharper here, adding a sting to the speaker's tone.
Consonance, in other words, subtly gives voice to the speaker's feelings: the reader can hear both his pleasure and his pain in the sounds he uses.