Would it be possible to get a higher quality read from using something like an Arduino's I/O pins and some bit-banged C code? I'd be curious to see what would be possible using cheap, off-the-shelf tools since a lot of people don't necessarily have an oscilloscope laying around. :P
Without looking up datasheets, just form the top of my head: the Arduino DAC most likely has 12 bits resolution (as common for cheap uCs), and maybe even slower sampling than a soundcard. A sound card was probably better than that even in the 1990s (say a Sound Blaster).