AFAIK, "lox" came to English through Yiddish, which got the word from German. And in German, the vowel was always "a" not "o" (e.g. https://ia600301.us.archive.org/32/items/etymologisches00klu... traces the word to "lahs"), so the vowel quality and the final consonant appear to have changed.
In Australia I have never heard the term - we goyim would just refer to a smoked-salmon bagel.
It's a /k/ in all "-achs", e.g. "Dachs", "wachsen", "Flachs", exactly as in "bochs" the emulator (compare "boxen"), except across boundaries, e.g. "wachsam, wach-sam" (wakeful, at guard), or contractions, e.g. "[Meister seines].Faches/Fachs".
although... if the language "Tocharian B" is extinct, how can we know its pronunciation? There must be a way, but I can't fathom it myself.
Speakers of Danish, Norwegian and Swedish can usually talk to eachother without major issues although the kids seem to be preferring to speak English nowadays.
Spoken Icelandic or Faraoese on the other hand? I usually manage to get a single word or phrase. The rest is gibberish.
The letter changed, but the pronunciation didn't - hence "not changed in sound". German "a" == English (short) "o".
You could go to the Eurasian steps in 3000 BC and ask for lox and they will know exactly what you mean.