I tried to do what you did but I can't see anything that would let me do what you did. Pulseaudio has a volume control but I don't think Pulseaudio is installed in Mint KDE. I looked at Systems Settings>Sound & Multimedia>Sound System>Hardware>Select the audio device. Pulesaudio isn't there.
I installed Gnome into the KED version of Mint. I haven't had any problems with the volume control in Gnome. May be there is some kind of problem with the volume control in the KDE version. I don't know.
I've found it's best to use the flagship version of a distro no matter which GUI desktop the distro uses. In the case of Mint that means using the version with Gnome. Installing Gnome into the KDE version is really the thin end of the wedge.
When version 6 is posted I think I'll do a clean install of the Gnome version.