Oh wow, I did not realize that floppy discs used the same exact encoding that was used on tape drives of the era, e.g. the TI-99/4A [1]. The wave file in the article is the exact same sound I'd listen to as a child, loading programs from cassettes.
Total coincidence -- I recently have been recovering games from my old TI-99/4A cassettes, also using an oscilloscope (the same model in fact; for no particularly good reason beyond it was the most convenient way to record off the only tape player I still own), Audacity, and two open-source tools to recover such data [2] and [3].
[1] http://www.unige.ch/medecine/nouspikel/ti99/cassette.htm