I'd say MP3 is a bit outside the realm of possibility within this universe. I know some settlements will be introduced possessing things such as electricity or even weak/low-grade motor vehicles. (Though didn't plan on introducing such things yet. Or for a bit.) But I feel the furthest i was thinking of going was radio's/walky-talky type devices, on that spectrum of things (If all that's considerable as on the 'same spectrum'.)