I did the quiz once and also got 4/6. I alternated through all the samples around 4 times before making each choice. I messed up on the only two where I felt baffled by the choices, so at least I know what I don't know. :-)
I chose the 320 kb/s for Katy Perry but was vacillating between that and the actual WAV. The one I completely screwed up was Jay-Z where I picked 128 kb/s. However, I knew I was lost with so much electronic glitching and distortion. While I heard differences, I had no idea what was a compression artifact versus a synthesis/mixing artifact. I also had no prior listening experience for either of these two, so no preconception for how it should sound.
For the other recordings, I think I could hear the masking of the psychoaccoustic models. Essentially, in the WAV I heard a "fuller" mix with overlapping sounds, while in the compressed versions I heard dropouts where a dominant sound was present in what felt like isolation. For Suzanne Vega this wasn't so easy, but I felt her voice and the reverb were more clear and natural in the WAV.
This was just with my old Sony MDR-NC22 noise canceling ear buds driven by my Thinkpad audio jack. My idea of a hi-fi environment is a Yamaha receiver and reasonable tower speakers like my Polks.