Acabei de encontrar isso: Hearing test on-line: sensitivity, equal loudness contours and audiometry
This hearing test measures the relative sensitivity of your ears at different frequencies. It produces equal loudness contours or hearing sensitivity curves – the frequency response of your own ears. The flash script allows you to play sound files with a range of frequencies and sound levels with the click of a mouse.
Deve ser bom para treinar o ouvido. Vou aproveitar e comparar meus resultados com o do resto do povo aqui em casa, para saber se estou tão mal de audição em relação a outros quanto imagino.
 Este outro site também é bom: Test your Hearing Cutoff
When I tried it on myself (I'm 39 years old), I found that with the system I'm using I can hear up to about 16kHz. My 6 year old son, using the same pair of headphones (Beyerdynamic DT 990 Pro) at the same volume, could hear up to 20.8kHz. If I turned up the volume, perhaps I could hear up to 17kHz, but definitely no higher.
There's a word for my condition: it's called presbycusis, which is defined as hearing loss associated with (but not necessarily caused by!) aging.
As one who's enjoyed audio all my life, I must say that I'm disappointed that I've lost so much of my hearing over the years -- I used to be able to hear the 19kHz hum of the high-voltage switching power supplies in TV's, but that's what happens as one grows older. Advice to fellow audio enthusiasts: buy the best equipment you can while you're still young!