most speakers attached to computers aren't going to play anything beyond 15khz with much authority.
This is a very cool link a friend sent. You can test at which frequency your hearing tops out. The sound test goes from 10 to 25 KHz.
http://www.ochenk.com/entry.php?id=63
Most older people cannot hear above 15 KHz so a dude in the UK made a device that emits a high pitch noise to put outside stores to drive away the loitering youth that tend to congregate there.
I can still just hear the 16 KHz but after that, it's all silent. Shantanubala, I'd be curious to know what your twelve year old ears can pick up.
-Kevin S.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic