This method works with XMMS and KDE desktop installed on distos I tested with k-2.6.*.*.

Open KDE control center/sound&multimedia/sound system and disable the current sound system. Hit apply button.

Launch XMMS and edit via options/preferences/output plugins from the current setting to OSS or Alsa, which ever is not active. Then open 'configure' setting for the new output plugin. Set the audio device to the USB headset settings. Adjust mixer card and mixer device settings accordingly. Click 'OK' button and then 'apply' & 'OK' before closing the 'options/preferences' buttons.

Open Kmixer and change the Current mixer settings to the USB headset. Volume settings and microphone settings will need adjusting as required.

Works with Streamtuner 0.99.99 and Skype 1.2.0.18
Skype requires changing settings under tools/options/hand-headsets from
/dev/dsp to /dev/dsp1 and saving.

I"m sure someone with more tech skills can find a 'slicker', simpler way to make these headsets work.

another crude sound/mic workaround:

every find yourself in need of a crude mic? plug a speaker into your mic port and talk into it. It really works

cheers