Thanks for the reply Fatmice!
What apparently relevant I did find under System Settings -> Advanced tab is that the khotkeys daemon indeed is running as a startup service.
I did not find much info on khotkeys that would be applicable here though, but I have looked through the khotkeysrc file and it does not appear to work on key scancode level. The general khotkeysrc shortcut format is "Ctrl+S", or "Alt+D", not "0x56" or "164", so I do not know how to tell it what volume up, down and mute is unless an intermediate app/daemon translates these.
Of course, since the keys are not recognized in the current X11/KDE setup, they can not be set as shortcuts in KMix Settings (my first bet), or in the System Settings -> Keyboard & Mouse -> Standard / Global Keyboard Shortcuts.
I have found two likely solutions, the "LinEAK" (lineak + klineakconfig) and "hotkeys" FreeBSD ports. Both claim to identify/translate the media/internet keyboard keys and presumably mediate between the hardware key scancode level and the X11/KDE key description ("Ctrl", "A", "VolumeUp").
Since I have never used any of those ports, I thought I'd ask around first to see if anyone did actually set up one of them or a similar port on this purpose. I hoped to avoid hitting a wall in the middle of a conf by getting some pointers on how to go about this thing. Someone always knows the more elegant way