I have heard of this, I think there was a thread on this posted here a few weeks ago. I can hear every tone on the site you listed. Oddly enought the tone I had the most trouble hearing was not the highest frequency but one of the middle ones, 17,000 I think. It just sound like a very high pitched screech, kind of like the ringing in your ears after going to a concert.
Jets, helicopters, and gunfire are not good for hearing I am noticing. I could hear the mosquito tone fine in my left ear...not so much my right, and above 16kHz I didn't hear much of anything...a little at 20K but could have imagined it it was so quiet.