In reply to:

If I'm not mistaken, aren't many cd's given a 20Kh ceiling, where there is no information above that point - and thus, no sound being produced at that point?

I don't think anyone uses a low pass filter to roll off frequencies above 20KHz, you're probably thinking of the Nyquist limit which requires at least two samples per oscillation of a frequency to "describe" it digitally.

ie: the highest sampleable frequency is half the sampling rate. (44,100 for CD / 2 = 22,050Hz)

Bren R.