This depends on your audio setup. Let's assume you basically just have alsa up and running and phonon (kde's abstraction of your sound system) connects to alsa. You can check if this is the case by looking at you kmix. When you open the mixer, you will see many channels. In this case, you'd just open a terminal and type in alsamixer. This will open up a text equalizer. Check if the relevant channels are turned up. You may find that one of the outputs is only turned up halfway or so. Use arrow keys to turn them up and hit esc to exit. To make this permanent, you'd (as root) do alsactl store and have the alsa daemon running at bootup to restore the volume levels. Even if you don't kmix will restore it in your kde session.
If on the other hand, you use pulseaudio (you should see only one channel in the kmix mixer in this case), the answer is a little out of my league. Technically, doing the alsamixer trick should still work, but I'm not completely aware of how pulseaudio relates to alsamixer settings. Try the alsamixer route and see if it fixes the problem. It's a good first step.