It is all relevant to your sound system. Some speakers cannot handle the higher tones so you dont hear anything or just hear clicks. Some sound cards clip sounds above certain frequencies. LCD built in monitor speakers are reletivly crap, as are laptop speakers.
Also the different encoding systems make the sound different, for instance on my system the I can hear the 21khz MP3 using winamp, but not using media player. Also I cant hear the WAV file using winamp but CAN hear it using media player.
If you use headphones and have noise cancelling turned on you wont hear some of the sounds. With head phones you will also hear more of the sounds.
According to the web site the tones are meant for an iphone, and so will be made to sound best on the crappy iphone built-in speakers.
I can hear all the sounds when transfered to my phone.
I'm 44 BTW.