I tried to mute a Skype recording channel using the popup from the system tray. KMix showed the volume change to zero and the "record" checkbox was disabled. Still the microphone was not muted in Skype. I opened pavucontrol, went to the recording channels tab and pressed the mute button there. The microphone was now muted in Skype. Reproducible: Always
*** This bug has been marked as a duplicate of bug 315920 ***