The sounds you're hearing are three oscillators.
Oscillator 1 is a sine wave at 200 hertz.
Oscillator 2 is a square wave at 100 hertz.
Oscillator 3 is a triangle wave at 50 hertz.
They're being generated in your browser with the Web Audio API.
* Keep in mind you need the latest version of Chrome, Safari or Firefox for this to work!