You have to add your user to the "audio" group, else you will experience error messages and silence when trying to play through the USB speakers. After logging off and on again, select the speakers in KMix (they show up as "C-Media USB audio") and make sure their volume is turned up. Launch the PulseAudio Volume Control. On the "Playback" tab, move any playing audio streams to the USB speakers.

Tested on Mandriva 2008 Spring.