Go to System Settings. Click on Multimedia. Click on Phonon.
There is a list of categories on the left side of the window. Click on the following options: Audio recording, Communication, Recording, Control (the last three are under the "Audio Recording" section which is a category itself). Move your device to the top of the list and click Apply.
Then make sure your microphone is unmuted: go to the KMix program and click the "Capture devices" tab. Make sure that the speaker icon at the bottom does not have a red X next to it; if it does, click the speaker icon.