You are probably right in what you say Ray most of us are aware of the limited frequency range of the human ear...but for the record I for one can tell the difference in audio quality between a good soundcard and a cheap one....and so can most of my music friends. It is not the range that is the problem but what frequancies are filtered out.
I can also tell you from first hand experience as anybody else probably can who has tried to make any decent recording using the standard inbuilt card on most computers....that it is not possible or worth the hassle.
Have you ever heard of air being added...well that is adding certain frequencies that the human ear cannot hear......it does make a big difference to the sound of other frequencies.......
so you perhaps cannot hear the air frequencies on their own but you can hear the differenc ethey make to the mix.
Each to his own taste and quality...but one thing I do know is that any demo not up to the specs previously mentioned in this thread will not cut it with record companies and most artists of any note......
Ray we are talking a couple of hundred bucks for a decent soundcard....that is nothing compared to time invested in songwriting or buying equipment...so do not sell yourself short or blow any chances by using a product not up to the task....take our word for it a standard soundcard is NOT up to the task for any serious audio applications.... including gaming....surround sound...or music production.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055