If the KMix icon is docked into the system tray, you can right click it to choose preferences. These items are:
minimize to an icon in the system tray
Just minimize KMix when closing the window, don't close it.
Show lines to mark positions on the sliders.
Display labels for each of the sound devices. Wether this item is checked or not, by holding the mouse over the icon for each device, you can see this information.