It is not that hard to make the volume keys work on systems without kmix.
Since pulseaudio is the default, pactl can be used to create the needed custom khotkeysrc.
An example is up and will be tested for the next ISO:
Or anyone can create the needed changes in that fille (it is ~/.config/khotkeysrc).
systemsettings > Shortcuts > Custom Shortcuts > right click in the middle column > New > Global Shortcut > Commad/URL
Give it a name, something like "Volume UP" > right column > edit the comment to something like "Increase system sound volume"
click the Trigger tab > assign the volume up key from your keyboard to it
click the Action tab > Command/URL and enter pactl set-sink-volume 0 +2%
Repeat those steps for volume down and mute, the needed pactl for down is:
pactl set-sink-volume 0 -2%
For mute it is:
pactl set-sink-mute 0 toggle
And hit apply