Learn English – What word(s) describe a woman’s voice as “sexy”

As noted in previous comments, uniliteral ("consisting of a single letter") and biliteral ("composed of two letters") appear in various dictionaries and as noted refer to single-letter and two-letter constructions. However, the etymology of literal (as at etymonline) somewhat restricts the scope of literal, making it less applicable to ideograms:

literal (adj.) late 14c., "taking words in their natural meaning" (originally in reference to Scripture and opposed to mystical or allegorical), from O.Fr. literal and directly from L.L. literalis/litteralis "of or belonging to letters or writing," from L. litera/littera "letter, alphabetic sign; literature, books" ... Meaning "of or pertaining to alphabetic letters" is from late 15c.

The meaning of previously-suggested monoglyphic is "having only one siphonoglyph, the sulcus, as certain polyps: contrasted with diglyphic", where a siphonoglyph is a "ciliated groove at one or both ends of the mouth of sea anemones and some corals"; that is, the word does not treat of single-character words. Grapheme, also mentioned before, has forms monographemic, bigraphemic, and digraphemic that are used apparently for speaking of one- or two-character Chinese/Japanese/Korean words. For example, Sproat et al, 1996 say:

Put another way, written Chinese simply lacks orthographic words. In Chinese text, individual characters of the script, to which we shall refer by their traditional name of hanzi, are written one after another with no intervening spaces ... Partly as a result of this, the notion "word" has never played a role in Chinese philological tradition, and the idea that Chinese lacks anything analogous to words in European languages has been prevalent among Western sinologists [but] twentieth-century linguistic work on Chinese has revealed the incorrectness of this traditional view. All notions of word, with the exception of the orthographic word, are as relevant in Chinese as they are in English, and just as is the case in other languages, a word in Chinese may correspond to one or more symbols in the orthography: K. ren2 'person' is a fairly uncontroversial case of a monographemic word, and ~] zhongl-guo2 (middle country) 'China' a fairly uncontroversial case of a digraphemic word.

The terms monographemic, bigraphemic, and digraphemic serve for alphabetic single-letter and two-letter words as well. Per wikipedia:

A grapheme is the smallest semantically distinguishing unit in a written language, analogous to the phonemes of spoken languages. A grapheme may or may not carry meaning by itself, and may or may not correspond to a single phoneme. Graphemes include alphabetic letters, typographic ligatures, Chinese characters, numerical digits, punctuation marks, and other individual symbols of any of the world's writing systems.

Maybe you are looking for singsong?

Merriam-Webster: "a way of speaking in which the sound of your voice rises and falls in a pattern".

I would say it's usually understood as an undesirable trait.

This video gives an (exaggerated) example.