"The Force..." is full of contradictions: creation-destruction, life-death, wet-dry. This idea of opposites, of contradictions, is reflected in the poem's sound. None of the poem's lines, in terms of content, seem very hopeful or joyous (even that nice flower in the beginning is attached to a "fuse"), but the lines are filled with energy and passion. Thomas's word choice has a great deal to do with this sound. Strong, active, single syllable words like "drive," "blast," "suck," and "whirl" run all through the poem. Even though the poem is talking mostly about the inevitability of death, it feels and sounds very alive.
Another thing that makes this poem sound so alive is the use of repetition. The repetition of phrases like "the force," "I am dumb to tell," and even the single word "how" repeated in identical spots in stanzas 2, 3, 4, and 5 gives this poem the lively feeling of a song or a some holy incantation.
There's also some alliteration at work in this one, particularly in the first line. The sound of this line sets the rhythmic, vibrant, and connected tone for the poem. Check out those repeated F and T sounds:
The force that through the green fuse drives the flower.
You can't dance to it, but the alliteration definitely helps to create a very lively sound, filled with bouncing energy. That vitality and passion in this poem's sound offers a nice contrast to the overall bleakness of the content—thank goodness.