KMix usage is straightforward. Every mixer device that your soundcard provides is represented by a volume slider. Mono devices have a single slider, stereo devices can have either one or two sliders, depending on your choice. Additionaly there is a panning slider at the bottom of the KMix window. If you have multiple soundcards, they will each have their own tab.
The volume sliders have a context menu, accessible by right clicking on the slider or device icon (at the top). Three entries are possible, but only those applicable are shown.
Mute or unmute the device
Show either one or two sliders. This is only applicable to stereo devices. The right slider controls right side volume, and the left controls left side volume.
Set this device as record source. Only applicable to recordable devices. This can also be done by clicking the red button below the slider (if it is available).