Consonance brings the poem's pleasant gardens to life on the page. Listen to the gentle sounds of lines 2-3, for example:
the gardens were well-tended
The fountains sprayed their usual steady jet;
"Were" and "well-tended" gently alliterate, while the sibilance of "fountains sprayed" and "steady jet" mimics the sound of that spouting water. All in all, these sounds create euphony; put simply, they sound nice—and that's exactly the problem! This place looks just as lovely as it did before the speaker's loss, having not changed in the slighted in response to the speaker's grief. Gentle /eh/ assonance ("well-tended" and "steady jet") adds to the lines' musical, lyrical feel.
There's more consonance in the second stanza, much of which is again sibilant. Listen to the /s/, /sh/, and /z/ sounds throughout lines 6-7:
The thoughtless birds that shook out of the trees,
Singing an ecstasy I could not share
These soft, hushed sounds again convey the gentle beauty of the scene at hand. The speaker's sharp, raucous pain seems utterly out of place in such a serene setting.
Line 12 dial-up this sibilance even more, though here it sounds threatening rather than peaceful:
That made your absence seem a savage force,
These hissing, spitting sounds convey the speaker's bitter anger at the fact that the world doesn't honor their grief. Try saying these lines out loud; that "savage force" sounds downright frightening!
The spiky /k/ sounds of the poem's final two lines stand out amidst the poem's gentle music. The consonance of "earthquake," "shaken," and "thinking" help to capture the speaker's turbulent emotions, those sharp /k/ sounds shaking the poem and evoking the sharp jolt the speaker feels when they think about the person they've lost.