This poem uses consonance to great effect. Along with its steady meter and rhyme, all this musical consonance allows the poem to practically sing! This is important in a poem stressing the value of lessons that, more than once here, take the form of birdsong.
The liquid /l/ sound is especially common, lending the poem a smooth, melodic, sensuous quality throughout. For instance, in the second and third stanzas note lush phrases such as "lustre mellow," "all the long green fields," "evening yellow," and "woodland linnet." In this way, the poem doesn't just convey nature's beauty as an idea, but also evokes that beauty directly through sound. This, in turn, reflects the speaker's argument: knowledge isn't just something to be acquired through books, and is better acquired by listening to nature's songs and rhythms.
In stanza 4, /l/ consonance gets joined by gentle /th/ consonance ("blithe," "throstle," "forth," "things") and a touch of sibilance ("throstle," "sings"). These additions bring yet more softness to this stanza, as if setting aside the world of books for sunlight and birdsong may thaw out the heart and ease the mind.
In the poem's final stanza, pairs of louder, bolder sounds add emphasis to the speaker's final call to:
Close up those barren leaves;
Come forth, and bring [...]
The sharp /c/ sounds and booming /b/ sounds make the speaker's words feel all the more urgent and insistent.