The consonant sound of the letter "s" is in every line. This is often called sibilance. In this poem, the sibilance creates a swooshing sound similar to a sleigh, or wind in the trees—think of the hushed sounds you have heard while alone in the snow. This brings up Frost's use of the "w" sound as well. It is not in every line, but when it is prevalent, it creates a certain mellow, calm mood. "W" is a "glide," and glides often flow into a vowel sound. The glides are easy and flow gently. However, friction begins to form in the second stanza as harder consonants are used. These hard consonants trip up the tongue, and I'm sure that Frost included these harder consonants is to show the struggle that exists below the gentle surface of this poem. The consonant sounds, along with a close reading of the setting and imagery, create the depth of this poem. Think about times when you have wanted to be alone in the woods; now couple that emotion/mood with the symbolism of the woods being the unknown or the subconscious.