It's a dystopia. It's always a dystopia. They let you do the weirdest things with every day stuff (please start a debate about that in the comments) and that makes them accessible and interesting.
So in this dystopia, as in all dystopias, people are deeply fucking bored. Always. They still remember the internet, or stories of it. Endless information piped from across the globe. The freshest content renewed with every f5. But now it's just smoke and craters for forever, and subsistence farming doesn't make you feel enriched when all your food tastes like shit. But even though dvd players and working tvs are hard to find and electricity hard to produce, mp3 players are everywhere. Every phone, every ipod, every stereo, every car radio with a cable jack or a usb port, it all gets used to endlessly loop the culture of dead civilisations.
But you get bored of hearing the same song over and over and over, especially when that song sucks, so obviously you trade them. With limited access to computers you can rarely copy an mp3, but you can trade one ipod for a stack of cds. All it takes is figuring out about what each song is worth. It turns out that is very hard.
Pop songs are clearly worth the least; they're common and lack depth. But, then, everyone likes them. They're guaranteed currency wherever you go, so that makes them worth more again. Rarer stuff is worth more to a collector, but who has time to collect in dystopia? You might be able to trade Vivaldi and Sigur Ros and the Sesame St theme with one of the Aquifer Queens, but if you're living rough in the badlands, Taylor Swift is better than gold.
And then there are the people that remember the stock market, and banks and mints and all the ways economies should work. So you get the one set of turntables in a thousand mile radius and go into business. Or you start printing banknotes, backed by the basement collection you found: every song, album and ep produced by The Beatles, or anyone that was in The Beatles, on cd, vinyl and live dvd. Or you just start speculating, telling people how many Adam Levine's they can get for a Drop It Like It's Hot remix. I'm not even sure that radio stations have a real-world allegory at this point. They definitely start wars.
Presumably, at some point, someone does just decide that bottlecaps have intrinsic worth. But think of life before that. How much is entertainment worth in a dead world?