Back in the days when everyone was into CDs and vinyl, acts sometimes hid Easter eggs within the packaging, the artwork, or on the album itself. The most common was the hidden/unlisted track, but occasionally someone would go above and beyond.
For example, Information Society, an alt rock-ish band from the late 80s put a fax message on their 1992 album, Peace and Love Inc. There was a track listed as “300bps N, 8, 1 (Terminal Mode or Ascii Download).” Play it on a CD machine and it as just noise. Play it through a phone to a fax machine and you ended up with a secret message. It didn’t make much sense, but still…
You can read the whole thing here.
But something even earlier has been uncovered. A band called Prodigal left a Commodore C-64 program to be discovered on a vinyl record released in 1984. To run the program, you have to record the audio onto a cassette and then load it into an old Commodore.