I’ve heard various reports about this, from the initial creation of a device that disperses crowds of young people, to the actual use of ring tones in class with teachers who couldn’t hear it, but this article has a sound file to download. When I played it on my computer, I could hear a very slight hum and that was it. The creator had to ask his kids (or grandkids) whether or not the sound was bad enough to chase people away. The New York Times says you have to be over 40 or so in order not to hear it, but other articles give the cutoff age as 20.
UPDATE: One person wondered just how degraded his hearing was, and he generated some test tones to find out. They are available for download here. My hearing is fine through 13,000 Hz then drops off sharply to the point where I can just barely hear 15,000 Hz.
UPDATE #2: Here’s a more recent link with tones at various frequencies. A friend of mine knows a lot about speakers, and he says part of the problem could be with my cheap computer speakers and not necessarily my hearing.