Probably someone throwing rocks at something, and instead of the normal clack-clack sound of rocks hitting other rocks, one time it went bing. And once they found out that one rock can go bing they probably started checking every rock to see if it might go bing too, or possibly bong.

I would think word of something like that would spread pretty fast, too. Like could imagine being a neolithic cave dude visiting the next village to trade some jewelry-quality bear teeth or whatever, only to see a guy playing a pile of rocks like a xylophone from the Flintstones? You’d be telling everybody about that shit. It’s pretty trippy to watch now; It must have been mind-blowing back when it was first discovered. I wouldn’t be surprised if it took on some kind of magical significance. And once you knew it was a thing you’d probably have your head on a swivel for the rest of your life keeping an eye out for some magic bing-bong stones of your own.