soundless (adj.)

"noise, what is heard, sensation produced through the ear," late 13c., soun, from Old French son "sound, musical note, voice," from Latin sonus "sound, a noise," from PIE *swon-o-, from root *swen- "to sound."

The unetymological -d was established c. 1350-1550 as part of a tendency to add -d- after -n-. Compare gender (n.), thunder (n.), jaundice (n.), spindle, kindred, riband, and, from French powder (n.), meddle, tender (adj.), remainder, dialectal rundel, rundle for runnel, etc. First record of sound barrier is from 1939. Sound check is from 1977; sound effect is 1909, originally live accompaniment to silent films.

The experts of Victor ... will ... arrange for the synchronized orchestration and sound effects for this picture, in which airplane battles will have an important part. [Exhibitor's Herald & Moving Picture World, April 28, 1928]