A bit of background: JRR Tolkien developed a linguistic branch he called "phonaesthetics", the study of beauty of words. Many writers have agreed on "cellar door" to be beautiful in terms of the sound of the phrase.
I've heard it said (perhaps apocryphally) that when non-English speakers are asked to judge words based purely on aesthetics, the word 'diarrhea' tends to rank quite highly.
Using generative Natural Intelligence, an italian wrote a top 40 song that sounded to 1970s non-english speakers as if it had had english lyrics: https://www.youtube.com/watch?v=-VsmF9m_Nt8
https://en.wikipedia.org/wiki/Phonaesthetics#Cellar_door