The first line of "Spring" has a nice lilt, but it otherwise sounds pretty normal. You could imagine hearing it in conversation or seeing it in an essay. With the second line, though, the language grows lush, along with its images. All that alliteration makes the words seem to twine together. The w-sound connects When / weeds / wheels, then that l-sound from wheels carries into long / lovely / lush. Then (we know, it just keeps going!) the sh-sound from "lush" curls into the word "thrush" at the start of line 3, and the l-sound picks up again in look / little / low, and then the word "thrush" comes in again and rolls right into the th-sound of through at the beginning of line 4. Whew! It keeps going, but you get the idea. The sounds are like vines, curling from line to line.
There's also an airy quality, a lightness to the language from all those vowel sounds and soft l and r-sounds in lines 2, 3, 4, 6, 7, and 8. We do get some harder, consonant sounds from "eggs look little" (line 3) "echoing timber" (line 4) and especially from line 5 with the phrase "strikes like lightnings." These hard sounds kind of serve as anchors. They keep us from floating away on all those light sounds. After all, the scene is heavenly, but we're still on earth. These are real eggs and real bird songs, and real physical emotions felt in response.
Then, at line 9, the rhythm really takes over, adding a new kind of tension to the language. Employing sprung rhythm (see "Rhyme, Form, and Meter"), the speaker talks in these bursts of stressed syllables: "What is all this juice and all this joy?" What might otherwise be a slow point in the poem (since it becomes abstract and rhetorical) is instead made very potent because of its rhythm. Through those bursts of stressed syllables, we get a sense of the emotion behind the question – how deeply important it is to our speaker. When we hit the hard sounds of "cloud, Christ, lord" in line 12, it almost sounds like someone is choking or sputtering. This is no neat little prayer you might say before bedtime, but something urgent created in a moment of deep feeling.
Finally, with the lofty o-sounds of "Most, O" in the last line, the music strikes a sort of middle ground. We're not in the lightness and lushness of those first eight lines, but the language has emerged from the thickest part of the speaker's turmoil, finding some relief, perhaps, in turning things over to God.