No sound from headphones after installing usb speakers

Hi All
I recently added usb speakers to my laptop setup - and they work fine.
However, I can no longer get sound through my headphones (though the
mic still works). If I select sound devices from control panel, and
choose the "audio" tab, I get a choice in audio playback between
realtek ac97 and the usb speakers. However, if I select the ac97
device I get no sound at all. Reselecting the usb speakers gives me
sound from the speakers. Ditto with changing the voice playback on the
"voice" tab. I have also tried setting the speaker type on the
"volume" tab to headphones - again, no dice.
Anyone got any ideas? (I should add that the headphones worked fine pre
the installation of the speakers).
Many thanks
Chris