The following describes briefly my own understanding of the concepts and choices around sound and how they affect sense.
Other writers would describe it differently, and probably better. Nevertheless everyone needs to understand these things in their own way, and incorporate them into their poetic practice deep enough to be (in part) almost without thinking. While I have separated these ideas out individually, in reality they work together so closely it's not clear where one finishes and the other starts. I find these are things best taken into account at the revising stage of writing, and over time if you continue thinking about them, hopefully good choices happen earlier and earlier in the drafts of your poems.
Sound can be examined at the following levels:
Individual words
Apart from the meaning, the sound of consonants and vowels contributes to the experience of the poem; and when the poet manipulates patterns of words to form sequences of sound like alliteration and assonance, this contribution increases. Alliteration and assonance can support or undercut the poetic form or genre, and contribute feelings of closure or relationship in unrhymed poems. Some simple examples are using letters like s, th, ck, sh, to indicate a physical sensation of sliding, stickiness, snake-like movement; or round vowels like o, ou, ow, to indicate a balanced openness. Where you may want to temporarily break free of a chosen form, these devices can preserve the general feeling of unity of the broken section with the rest of the poem, such as Milton's use of the following words in a description of Hell, where he breaks the blank verse meter he is using:
Rocks, Caves, Lakes, Fens, Bogs, Dens, and shades of death
(Paradise Lost, 2.621)
I find the best way to begin being aware of the feelings associated with consonants and vowels is to find the words you already use and examine the effect they have. An example is the phrase I used above: a balanced openness.
The balance is expressed in the sound by the two 'a''s either side of the 'l'; in addition the heavier stress on the first syllable is "balanced" by the longer although lighter second syllable. The past tense use of the word by adding the letter 'd' means the word does not end with a trailing off sound of 'nce' but the complete ending of 'd'.
Openness obviously starts with the wide mouth of the 'o' which rounds down into the 'pen' and stretches out with the 'ness' to make it more than describing the quality once or of one finite object. The repeating of the 'e' vowel sound helps bring to the second and third syllables a continuation that also contributes to the effect aligning with the meaning of the word.
Combining these two words as a phrase, therefore, combines these two experiences of sound to create a specific effect that may or may not fit into the poem you are creating.
Whether this applies to languages other than English I do not know, but generally I have found that the sound of individual words (nouns and adjectives especially) often can be felt to contribute to our experience of their sense.
Individual lines
This is where meter comes in explicitly. Above I started to talk about combinations of sounds within and between words. Looking at a line of poetry, the alliteration and assonance within words and between words is one layer of sound; the other is the manipulation of meter (and by association, line length). A poem being in free verse does not mean there is no meter or the metrical effect does not matter. It simply means the poet has no explicit grid of meter to write on top of that is universally recognised; the poet may have - often does have - their own habitual regular or irregular meter(s).
The sound of individual lines by meter can be harder to assess because it is a subtler effect and can be drowned out by alliteration and assonance. I find it useful to think of the background meter as the drumbeat underneath the clash of cymbals (alliteration and assonance), and the flowing melody (actual words chosen).
I'll only discuss this briefly, because there is so much written on the use of meter to contribute to the effects of poetry (see Shaw's Blank Verse text as an example that focuses on one poetic form).
In non-free verse, the meter is an externally imposed idea that the actual poem must treat with but not completely live up to, so that its fluctuations become expressive and not monotonous or robotic.
Continuing with the blank verse examples (as this is my favourite form), substitution of any of the five "feet" of the iambic pentameter line alters the expected meter and helps express what may be happening in the poem, or sometimes in the relationship between the reader and the poem.
Keats with his exquisite sense of metrical manipulation provides any number of useful examples of different forms. One of my favourites of his, Ode to a Nightingale, includes these lines when talking of his morbid invitation to death:
Still wouldst thou sing, and I have ears in vain -
To thy high requiem become a sod.
The two strongest metrical effects here, in my opinion, are the substitution of the first iamb foot of the first line with a trochee, emphasising the 'Still' over the 'wouldst', as the strong answer to his preceding question, and in the second and third iamb feet of the second line, trochees again, this time expressing the sense of rising with the word 'high' and the falling of from here through the 'requiem' emphasising falling down to (literally) earth with the word 'sod'. In terms of word choice, the sybillance of 'still', 'wouldst', 'sing' and 'sod' both approach the melodious idea of the nightingale's song, and form links between these words that are so important in the meaning Keats is making. The rhyme of 'thy' immediately preceding 'high' increases the effect of the trochee substitution.
With free verse, the reader must go with the poet's meter and be thrown up and down and sideways as the poet chooses (rather like the expressiveness of classical music compared to the regular beat of pop music).
The long and flexible lines of Whitman in his deservedly adored Song of Myself shows how this can be well done.
Again examination of Whitman's meter has been done in more detail elsewhere, so I will just note that his changes of metrical emphasis can match his meaning, such as in the last line of the first section of this poem:
Nature without check with original energy.
I interpret the sound effects as follows. The stresses on the words 'without' and 'with' are arranged in the line so that the stressed 'out' of without appears stronger after the unstressed 'ure' and 'with' preceding it; and later in the line, 'with' appears a little stronger by preceding the (again relatively) unstressed 'o'. The repetitive use of the 'g' in particular, in the words 'original' and 'energy' help combine and heighten the focus on this phrase.
Stanzas and overall poem:
The sound effects of groups of lines and the poem as a whole rely on the work done we have just discussed - individual word choice, and relationships between words in terms of stress and alliteration/assonance.
Looked at in view of a number of lines, the progression of the reader's expectations of meter, and their experience of the variations or effects of sound becomes cumulative, and this can be used in terms of progression, and unity.
Progression of meaning of the poem can be matched and expressed also in the progression of sound effects. For example, moving from a regular, lightly-stressed, buoyant stanza full of o's, l's, n's, r's, etc, to a more irregular, highly-stressed stanza using u's, z's, hard g's, ck's etc, starts to create a story, and the thoughtful poet will have done this deliberately, perhaps to reflect a change in feeling in the meaning of the poem, an event or a change of speaker, or perhaps how much we should rely on the sincerity of the words.
At this stage examples can become time-consuming to explain, and probably unnecessary anyway. I would suggest taking a look at some sonnets of Shakespeare for a manageable hunk of words.
You may be asking "where are the contemporary examples?". Go look at your favourite contemporary poems and figure out how their sound affects their sense. One of the tests of good poetry, in my opinion, is whether the sound actually contributes to the sense, or seems random, with opportunities for emphasis lost.
