Thank you very much for your quick response! Following this advice, and also one that I found in the web:
http://dev-loki.blogspot.com/2006/04/ma ... odmap.html
I assigned keycodes for the Fn-keys in an .Xmodmap file, and then used KHotKeys to activate them.
The problem is, it still has no effect on the volume output, although there appears on the screen a small pop-up window when I use the Fn-F6 -- Fn-F8 keys, showing the action I'm taking. The KMix applet also changes its ststus, only the volume output is not affected!
Can you suggest what I've missed, or done wrong? I can provide more details if needed. Thanks again for your attention.