You can control some MIDI control change, volume, expression and others. KMidiMixer has 17 sliders. 16 sliders are for each MIDI channel. Right most slider is for master volume.

Each 16 channel slider has check box. It's a mute/solo switch. Just click makes each box on/off. Alt-click makes clicked box on and others off. Option-click makes clicked box off and others on.

You can change background bitmap. Just drop any image file onto KMidiMixer window. ALT-B clear background bitmap.