(even surpassing the role of any visuals, although practically this is an impossibility, because soundtracks don't exist in isolation)?
well they do! :) "War of the worlds" (the radioplay from 1938) had an enormous effect on society. Never before was a medium able to scare so many people (to the actual point of fleeing the city, some say).
Whether it was the story or the sound design, i'm not certain. Sound design, wasn't 'around' back then. Although many will disagree :)
I'll think a bit more for other examples, great question!
Some more examples, besides the obvious signal sounds:
Car (the sound of doors closing, etc). Associaton='safety,transport,luxury (bmw, mercedes)'
Harley exhaust (everyone recognizes that in the western world). Association='bikers,freedom,route66,easy rider'
Senseo coffeemachine (designed by Philips, has a particular sound to the pump). Association 'coffee!, more coffee!, personally i detest senseo coffe.