I suspect that's more due to aliasing or some other DSP that's happening (most audio designs have bandpass filters, because anything outside the usual range is not considered a valid signal.)
While this is true, 19k is also hearable to most young people (maybe under 25 or so). We use 44k because it is right above double the cutoff of human hearing.
Only exotic, multi-thousand dollar speakers will reproduce 44k, and for that you need a 96k sampling rate. By using a sample rate of 44k you can theoretically only reproduce up to 22k.
You may have misunderstood my comment. 19k/20k is right at the limit of human perception for people with extremely good ears. Thus, "we use 44k because it is right above double the cutoff," ie 22k.
As an aside, you will see 96k used (or more commonly 48k), but mostly for production work where the extra information could have value (comparable to using RAW for images, even though humans can't perceive all the information "natively").
Probably because while the signal is carried at 19 kHz, its bandwidth is wider. Assuming it's 6 kHz wide would make it 16-22 kHz. You can't send data that is just 19 kHz because it's just a static sine wave.
