Thanks for responding!
Yes - kmix has no problem starting when I load it manually, and I've checked all settings to make it go to system tray and start automagically.
What puzzles me is that it was present for all system users, and for no apparent disappeared for each and every one. While I would very much like to fix the problem, I am even more interested in learning exactly why it disappeared as to prevent this from happening again, and avoid possible memory leaks.
Is there any way to debug whether kmix STARTS as supposed to when KDE login loads, but then is terminated due to problems, or if it simply does not start at all?