Frost's form matches the content of his poem. He is writing about the intricate designs of a divine creator and so he writes with a very precise sense of control over his language. That comes through in the sound of the poem, too.
We talk about form and meter in the appropriately-named "Form and Meter" section. As for the sounds you're hearing in his poem, there are a couple of interesting moments with alliteration. First, we've got line 7, which gives us "a snow-drop spider" and "a flower like a froth." By alliterating those S and F words (as well as using lots of consonance with those P and D sounds), these two content images become lovely and light. This makes the contrast of the awful scene just that much clearer to us. Secondly, we are given the alliterative "design of darkness" in the next to last line. Such a label gives this possible divine force more power and personality.
More than that, the kind of sound play that we hear in this poem is just another reminder of the poem's Designer. All these sounds don't happen just by accident. We're experiencing a well-wrought work, and the sounds of the poem underscore that effect.