...even at just the CD level of recording at double the 20kHz range of human hearing...
... how can frequencies beyond the range of human hearing improve the sound recording ¿
For digital systems, the upper limit of the frequency response is determined by the sampling frequency. The choice of sample rate used in a digital system is based on the Nyquist-Shannon sampling theorem. This states that a sampled signal can be reproduced exactly as long as it is sampled at a frequency greater than twice the bandwidth of the signal. Therefore, a sampling rate of 40 kHz would be theoretically enough to capture all the information contained in a signal having frequency bandwidth up to 20 kHz.
Basically, pure, single frequency vibrations, have one positive and one negative peak per cycle. A twenty kiloherz vibration will therefore have a total of 40 thousand peaks and troughs. So you want a sampling rate a little higher than that to get a reasonable representation of the sound. Remember that twenty kiloherz is the extreme value and lower frequencies will be sampled more than adequately. Even people who have ears that can detect high threshold frequencies are not necessarily perceiving them very accurately anyway. Who can claim that their brains can truly pick out the odd awkward sample out of 44,100 samples within one second? Especially with the only awkward ones occurring at frequencies above 10,000 Hz, with very low odds of occurring, even towards the high threshold frequencies.
If you've got a shitty recording on a CD, it's not because of the digital sampling process.