So sometimes at junk/antique/thrift stores, I rifle through those ubiquitous boxes of caseless cassettes. And sometimes I'll buy the blank looking ones, especially if they're not rewound because then I know there's at least SOME content on them.
I've never found anything particularly gripping, but it's kind of interesting to hear what anonymous people find worthy of recording. I found one from the 80's with a Celtics playoff game on it. Haven't listened to it, not yet.
One of my favorite weird non-blank cassettes is a Dexter Shoes promo cassette which features Suzyn Waldman singing a song about Ted Williams to the tune of "Take Me Out to the Ballgame". It's completely awful and littered with phrases like "Splendid Splinter" and so forth.
I'm cleaning my apartment and really digging for gold and finding aged weirdness. I hooked up my NEW record player and guess what? I can't get any sound.