If you were to read this poem out loud (go ahead and try it right now, Shmoopers—we'll wait right here), you would hear a lot of pleasing sounds. Not only is the speaker describing a beautiful scene among these trees, but the sounds created by his word choices just add to the beauty.
For example, check out all the alliteration going on in these lines. We have B words in line 2 ("bloom along the bough"), W's in lines 3-4 ("woodland ride / Wearing white"), and S's in line 7 ("seventy springs a score"). Now, in a poem of only 12 lines, that's a whole lot of words joined by their starting sounds. So, what gives? The pleasant effect of having these words joined together—the way they roll off the tongue in connected fashion—subtly underscores the beauty of the scene that the speaker's describing.
But he's not done there. Check out the assonance in places like line 1: "Loveliest of trees, the cherry now." All the long E's in that line create a pleasing echo in the mind's ear (if you can picture that). The same goes for all those long I sounds in lines 3 and 4 ("ride," "white," "Eastertide") and the long E's in lines 7 and 8 ("seventy," "leaves me fifty").
Just like those pretty cherry trees, the sounds of this poem are, well, pretty. We get chimes and chains that show how our speaker is not just finding beauty in what he sees. He makes sure to add beauty to the telling as well.