Comment 4 for bug 358168

Workaround:
System settings >> Keyboard & Mouse >> Global Keyboard Shortcuts >> KMix
There is a keyboard shortcut configured for the action "Mute" (don't know the real name, on my german system it is "Stumm"), the key configured for this action is named "Volume Mute", but this action has no effect, it only displays the volume level on the screen. Below the "Mute" action there are several actions beginning with something like "Muting on/off" (german: "Stummschaltung an/aus").
Selecting "Muting on/off - Master, HDA Intel" or "Muting on/off - PCM, HDA Intel" and pressing the Mute button works for me. It shows nothing on the screen, but it sets the corresponding mixer Channel to mute.

Nice would be an action "Muting on - Master, HDA Intel", not "on/off". This would have the advantages, I described in my previous post.