For anyone wondering, the current oldest playable audio recordings were produced by the phonautograph (1857)[1], which was made for studying acoustics. They were write-only until 2008, when they became playable via computers. There's a sample on Youtube. [2] [1] http://en.wikipedia.org/wiki/Phonautograph [2] http://www.youtube.com/watch?v=s0fhEpxrFvo
The phonautograph was essentially a mechanical audio oscillograph. It graphed the sound waves on paper by conducting the acoustic vibrations to a stiff bristle up against lamp-black blackened paper wrapped around a slowly moving hand-cranked drum. The white traces left behind allowed properties such as frequency and amplitude to be estimated.
I wonder if its inventor, Édouard-Léon Scott de Martinville, ever contemplated the problem of reproducing the graphed sound.