"systematic vowel alteration in the root of a word to indicate shades of meaning or tense," a characteristic of Indo-European languages, 1845, from German Ablaut, literally "off-sound" ("off" here denoting substitution), coined by J.P. Zweigel in 1568 from ab "off" (from Old High German aba "off, away from," from PIE root *apo- "off, away") + Laut "sound, tone" (from Old High German hlut, from Proto-Germanic *hludaz "heard, loud," from suffixed form of PIE root *kleu- "to hear"). The word was popularized by Jakob Grimm and Franz Bopp. The process is what makes strong verbs in Germanic. An example is bind/band/bond/bound + (German) Bund. Compare umlaut.
In our language, it seems to us that the uncouthness of such compounds as Upsound, Offsound, and Insound, could hardly be compensated by any advantage to be derived from their use; and we therefore purpose, in the course of this work, where any of these terms occur in the original, to retain them in their German shape. Of these terms, Ablaut and Umlaut are those which chiefly, if not alone, are used by our author. [from footnote in translation of Bopp's "Comparative Grammar," London, 1845]