Any class or type of sound which once existed, but has since been replaced, superseded, or has otherwise ceased to be heard except as a museum artifact. Disappearing sounds are generally those associated with social activity, although some natural species and their sounds have also become extinct. When the energy forms used in society change, many direct and indirect acoustic changes result. As travel by horse gave way to mechanized transport, the sounds of shoed horsehooves, reins, sleighbells, and wagonwheels disappeared, but so did the sound of the blacksmith and wheelwright.
In some cases an interesting morphology occurs if the characteristic sound undergoes a metamorphosis, rather than simple extinction. The development of the hunting horn and post horn, for instance, through the steam whistle and compressed air horn to the modern, electrically driven devices is a typical such progression. Sometimes, changing materials form the basis of acoustic change: earthenware to glass and then plastic, for instance. Road surfaces show another interesting progression with a strong acoustic component.
Often disappearing sounds become sound romances, and if recalled, as for instance in earwitness accounts, or through recordings, may trigger extensive memories of the past. In many cases, the sounds involved did not seem important to the person at the time, and their actual passing may have gone unnoticed. One important activity of soundscape studies is the careful collection and archival documentation of disappearing sounds.
(Source - Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)