Xylophone opens with x, which English will not pronounce as the Greek ks at the start of a word and so reduces to a plain z sound. It is a Greek marker of the same kind as ps- in psalm and pt- in pterodactyl: the letters are faithful to the source, the pronunciation is not.
The word is Greek in both halves — xylon, "wood," plus phōnē, "a sound," complete with the ph of phi — and entirely Victorian in assembly, coined in 1866. Early accounts sometimes Englished it as music-sticks, which did not take. The first English notice of the instrument is charming: The Athenaeum in April 1866 reported a seven-year-old prodigy doing wonderful things with little drumsticks on "a machine of wooden keys, called the 'xylophone'," noting that his rapidity was amazing and the sound of his instrument unequal.
xylophone (n.) musical instrument consisting of a graduated series of wooden bars, sounded by hand-held wooden hammers, 1866, coined from Greek elements: xylon "wood" (see xylo- ) + phōnē "a sound" (from PIE root *bha- (2) "to speak, tell, say"). In early accounts occasionally Englished as music-sticks . Related: Xylophonist . Then there was Signor Tamburini singing astonishingly well; further, a prodigy, seven years old, who does wonderful things with little drum-sticks on a machine of wooden keys, called the 'xylophone,' almost five octaves in compass. His rapidity was amazing; the sound of his instrument is unequal. [The Athenaeum, April 7, 1866] also from 1866
xylophone came in through Greek, and its spelling follows that family's conventions.
How that family writes these sounds
| a bare /n/, /t/, /s/, /z/ at the start | put the silent letter back on the front: pn-, pt-, ps-, mn-, x-pneumonia, pterodactyl, psalm, mnemonic, xylophone |
What gives it away on the page
| ps-, pn-, pt-, mn-, x- | Initial clusters English can't actually pronounce, so it silences the first letter.psalm, pneumonia, pterodactyl, mnemonic, xylophone |
Placing it without the spelling
| a medical or technical word starting with a bare /n/, /t/, /s/, or /z/ | Greek — the letter you can't hear comes firstpneumonia, pterodactyl, psalm, xylophone |