I could say that it’s the big problem with the artistry of poetry, but I figured I’d ruffle enough feathers as it is. No matter how I say it, however, all of these literary devices rely on how people say words, and that’s something that is simply not consistent.
When writing, we have little choice but to rely on how we pronounce words. Someone from a different region, however, might pronounce the vowel or consonant sounds differently from how we do. For example, the midwest United States tends to pronounce a “t” in the middle of a word as a “d” (“liddle” instead of “little”). That means words that have consonance for people from that region may not for people somewhere else. Vowel changes are even more drastic from region to region, affecting assonance and rhyme.
The other problem with pronunciation changes is that they can shift where the emphasis is placed on syllables. For two-syllable words in English, the first syllable is usually emphasized for nouns and adjectives while the second syllable is stressed for verbs. That doesn’t mean that everyone is going to say it that way, which can make all the effort you put into the meter of your poem feel wasted (just wait until you hear someone say it completely wrong).
Sure, the dictionary gives rules for pronunciation, but no one’s going to check every word in the dictionary while writing. Even if you did, people aren’t going to speak that way simply because the dictionary said to, so why bother?
Instead, resolve yourself to the fact that people may not get the full effect of the meter, assonance, consonance, and rhyme that you intended. Whether they’re from a different region, or whether the pronunciation has simply changed since you wrote the poem 20 years ago, there’s no guarantee that it’ll be read as intended.
All we can do is write as well as we can and hope that our writing will reach others – whether the literary devices come through or not.