KVolumeOSD is a simple KDE application that runs in the background and responds to volume buttons by showing a little OSD.
As usual, run:
make install (as root)
Make sure KMix is running and that no other KDE application is set up to respond to the XF86AudioRaiseVolume|LowerVolume|Mute events. Then run 'kvolumeosd' to start.
You can also select KVolumeOSD from the K-Menu or run 'kvolumeosd --configure'. That will bring up a little configuration window that allows you to adjust the look and behavior of KVolumeOSD to your likings.
Finally you can run 'kvolumeosd --quit' to close the app.