It is very easy to generate tones with the ZDev library and the ZKit-51. This project shows how to generate tones of various frequencies using the PWM controller.
There are variety of user input devices for embedded systems, like buttons, switches, knobs, sliders, etc. Sliders are most commonly used for volume controls. This project shows how to interface a slider to the ZKit-51, and how to use it as an input device.