Assonance

Assonance is the repetition of vowel sounds to produce dramatic effect. In the Raven, Poe uses the long "o" sounds in stanzas such as-

But the silence was unbroken, and stillness gave no token,
and the only word there spoken was the whispered word,
Lenore

and repetitions of "a" sounds —

...tell this soul with sorrow laiden if, within the distant Aiden,
it shall clasp a sainted maiden...

Poe maintained that "o" and "r" are the two most powerful sounds in the English language. Hence the Raven's famous refrain, Nevermore.

Assonance is a subtle effect, making prose more poetic. Be careful of too much, or of rhyming, which can get distracting to the reader.