wouldn't they have been recorded when the music was played originally, and then the harmonic itself be played by the speakers.
Again, my understanding only, I'm not an expert.....
I don't recall ever actually hearing the difference tones recorded. I've only experienced it live (both playing and listening). I did see a couple of 'net references that theorize that is has to do with how the human ear responds to the two tones. The recording mikes would not necessarily respond in the same way so may not pick it up.
I honestly did not think human hearing extended beyond 20K at least appreciably
And again if I recall from college, 22K is the limit.
And back on topic a bit...I'm surprised at all that there are speakers in these ranges. Maybe I'm not looking at high end enough speakers, but it seems to me that all of the speakers I've looked at are at least 20-20K, with some going down to 15 - 20K. I can't recall seeing any that only go to 15K.