Phonaesthetics

Phonaesthetics (from the φωνή, phōnē, "voice-sound"; and, aisthētikē, "aesthetics") is the claim or study of inherent pleasantness or beauty (euphony) or unpleasantness (cacophony)  of the sound  of certain words and sentences. Poetry is 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, ).

From this meaning should be distinguished the closely related but different concept of phonaesthesia, which does not refer directly to aesthetic attributes of sound, but to phonetic elements that are inherently associated with a semantic meaning. The term was introduced by J. R. Firth in 1930 "The phonæsthetic habits [...] are of general importance in speech." Firth defined a phonaestheme as "a phoneme or cluster of phonemes shared by a group of words which also have in common some element of meaning or function, though the words may be etymologically unrelated."

Sub-phonematic euphony
In most languages, phonetic combinations which are difficult to pronounce will be adapted to allow more flowing speech, for reasons of ease of pronunciation rather than aesthetics. These adaptations will be sub-phonematic at first, but over several generations will lead to phonematically relevant sound changes. Most of the euphony or mellifluous design of a formal language is pure coincidence, yet phonaesthetics relations with meaning can arise to frequent use and may even become cliché.