maybe you should check if any other program using sound is opened at the same time than audacity? I know it's weird, but in both of my computer audacity got recordings troubles when other programs are using the sound server (even firefox). If I just close them and restart audacity, everything runs fine.
and off course you have to enable microfone in kmix
edit: forget this last line, I didn't see it was a radio recording, and also it's gonna be difficult to close everything...