It is a combination of the actual and our perception of it IMO. Even if the wave was a square, our ear hair wouldn't pulsate as square because of the inertia. And as such we wouldn't perceive a perfect square any differently than a "wavy" square, which is why Nyquist theorem/Fourier transform works. My comment was about understanding why the sound becomes richer just by flattening a wave. It is not intuitive that this would be the case but in essence it's as if you are _adding_ more frequencies that didn't exist before to the same sound. More frequencies are interpreted by the ear/brain as more instruments or richer instruments.