I was having problems recording with KDE 3.3 (in Suse), until I discovered I had misconfigured the Kmixer.
Make sure that the "Capure" input device is enabled (indicated by a red-led-like icon at the bottom). Then your input device, i.e. mic, should also be enabled with a corresponding light. Finally, the green light for you mic should also be on (there is no green light for the capture device in my Kmix ver 2.1). Hope that helps!