Laudator Temporis Acti

J.R.R. Tolkien, O'Donnell Lecture on English and Welsh:

Most English-speaking people will admit that cellar door is 'beautiful,' especially if dissociated from its sense (and from its spelling). More beautiful than, say, sky, and far more beautiful than beautiful. Well, then, in Welsh for me cellar doors are extraordinarily frequent, and moving to the higher dimension, the words in which there is pleasure in the contemplation of the association of form and sense are abundant.

Credit for the notion that cellar door is beautiful or musical has been given to several different literary figures, including Edgar Allen Poe, Dorothy Parker, H.L. Mencken, F. Scott Fitzgerald, et al. Unfortunately, those who give credit seldom if ever cite chapter and verse. Here is some actual evidence, in reverse chronological order.

A Mencken Chrestomathy (New York: Knopf, 1949; rpt. New York: Vintage Books, 1982), p. 459, with reference to a column by Mencken in The Smart Set (June 1920), pp. 138-143:

Poetry, in fact, is two quite distinct things. It may be either or both. One is a series of words that are intrinsically musical, in clang-tint and rhythm, as the single words cellar-door and sarcoma are musical. The other is a series of ideas, false in themselves, that offer a means of emotional and imaginative escape from the harsh realities of everyday.

Alma Blount, Intensive Studies in American Literature (New York: Macmillan, 1914), pp. 30-31:

Poe, who studied sound effects carefully, says that he chose "Nevermore" as the refrain for The Raven largely because the word contains the most sonorous vowel, o, and the most "producible" consonant, r. An amusing story is told of an Italian lady who knew not a word of English, but who, when she heard the word cellar-door, was convinced that English must be a most musical language. If the word were not in our minds hopelessly attached to a humble significance, we, too, might be charmed by its combination of spirant, liquids, and vowels.

"The Spectator," The Outlook 93 (September 4, 1909) 16-18 (at 18):

The Spectator heard, not long ago, of a Spanish gentleman who politely disclaimed for his own language any monopoly of musical words, saying, "What word in Spanish is more musical than your own term 'cellar door'?" And, indeed, if, instead of being a term suggestive of ashes and dilapidation, the same sounds, Celadore, were, let us say, the name of a beautiful heroine in a play of Shakespeare's, we might readily understand how the ear might find them musical.

Cyrus Lauron Hooper, Gee-Boy (New York: John Lane, 1903), pp. 43-44:

Even into advanced manhood he remembered with approval these experiences, and had no sympathy with those unmusical souls (God save the mark!) who see no beauty in a word. He even grew to like sounds unassociated with their meaning, and once made a list of the words he loved most, as doubloon, squadron, thatch, fanfare (he never did know the meaning of this one), Sphinx, pimpernel, Caliban, Setebos, Carib, susurro, torquet, Jungfrau. He was laughed at by a friend, but logic was his as well as sentiment; an Italian savant maintained that the most beautiful combination of English sounds was cellar-door; no association of ideas here to help out! sensuous impression merely! the cellar-door is purely American.

I haven't been able to find any example earlier than Hooper in 1903. It's noteworthy that Max Beerbohm, without mentioning cellar door, made the following observation a year earlier, in "The Naming of Streets," The Pall Mall Magazine Vol. XXVI, No. 105 (January 1902) 139-144 (at 141):

What you take to be beauty or ugliness of sound is indeed nothing but beauty or ugliness of meaning. You are pleased by the sound of such words as gondola, vestments, chancel, ermine, manor-house. They seem to be fraught with a subtle onomatopoeia, severally suggesting by their sounds the grace or sanctity or solid comfort of the things which they connote. You murmur them luxuriously, dreamily. Prepare for a slight shock. Scrofula, investments, cancer, vermin, warehouse. Horrible words, are they not? But say gondola—scrofula, vestments—investments, and so on; and then lay your hand on your heart, and declare that the words in the first list are in mere sound nicer than the words in the second. Of course they are not. If gondola were a disease, and if a scrofula were a beautiful boat peculiar to a beautiful city, the effect of each word would be exactly the reverse of what it is. This rule may be applied to all the other words in the two lists. And these lists might, of course, be extended to infinity. The appropriately beautiful or ugly sound of any word is an illusion wrought on us by what the word connotes. Beauty sounds as ugly as ugliness sounds beautiful. Neither of them has by itself any quality in sound.