Phonaesthetics is the claim or study of inherent pleasantness or beauty (euphony) or unpleasantness (cacophony) of the sound of certain linguistic utterances. Poetry is often considered euphonic, as is well-crafted literary prose. Important phonaesthetic devices of poetry are rhyme, assonance and alliteration. Closely related to euphony and cacophony is the concept of consonance and dissonance.
The phrase cellar door has some notoriety as the reputedly most euphonic sound combination of the English language (specifically, when spoken with a British accent).