Try going to YaST > Hardware > Sound > Other > Volume and move both the PCM and Master volume up very very high. Then close YaST. Then open your mixer, and adjust the volume there with PCM and master volume such that the level is satisfactory.
Note, every time you start a multimedia app, your PCM volume control may dynamically change, and you may need to go back to your mixer to adjust the PCM volume.
In openSUSE-11.0 I used KDE-3.5.9, and it worked great. I have subsequently upgraded our 11.0 PCs (2 desktops and 1 laptop) to 11.1 with KDE-3.5.10, and they are also working great. KDE-3.5.10 is the best KDE yet IMHO. Our two 10.3 PCs (1 desktop and 1 laptop) are still running KDE-3.5.7 and are also running well.