I was curious to see how audible square wave duty cycle would be (I figured not much) but sadly the audio examples are clearly broken on this page--choosing the same one multiple times gives totally different sounds.
Changing the duty cycle of a square wave is called Pulse Width Modulation and is an extremely audible and iconic sound. If you have ever heared music any on the Commodore 64, you will familiar with its sound. PWM is also available in many professional synthesizers of the same era.
Yes! My dad has an old Commodore 64 and I vaguely remember a skiing game. I think it's so cool what folks were able to accomplish musically on such limited systems at the time