[acarabott]

I made a quick fork to add audio to this using the Web Audio API (webkit only, tested on Chrome and Safari).

http://acarabott.github.com/teoria/

[SoftwareMaven]

Had fun figuring out where mine and my wife's hearing fell off. Now I know where to put it when I want my kids to bugger off. :)

[cosmikduster]

http://onlinetonegenerator.com/ has a nice hearing test using the audio API of Firefox.