Here's a pretty neat example of technological time travel: media preservation folks at Indiana University found gramophone records printed into books from the late 19th century. The records aren't solid media. They're just ink-based reproductions of the grooves found on records. From these grooves we can produce waveform images and from the images we can produce audio. The original authors of these reproduced records explicitly intended their work to be accessible in the future, though they clearly didn't foresee technology like scanners or audio editing software.

There are other, earlier examples of hand-engraved waveforms. Here's one from 1806, mentioned in the link above but looped by me just for fun: