I skimmed the articles and bookmarked them for more thorough reading later, thanks very much!
The Wiki stub seems to be the only one that actually delves into ultrasonics unless I missed it? I'm from Missouri; do these show me that we can actually hear (i.e. something in the ear responds) at 40 kHz? If e.g. the hair cells move enough, I could understand that they (or the brain) might mix down, but if their sensitivity is nil at that high a frequency then I am not sure it matters...
I am leaving aside the probability of one's speakers generating that high-frequency energy as irrelevant to my fundamental question. Nor is the question of the high frequency content of keys jangling, cymbals, instrumental overtones, etc. in doubt; just my ability to hear them!
The highest my hearing was ever tested was 22 kHz whilst I was in college, but judging my my more recent loudspeaker tests whiole setting up my system it's in the 10 - 12 kHz region now (at age 51).
BTW, how did we get to this from jitter?