no sound to the headset!!!

my headset won't work even though I select it from the list. It's a plantronics USB based headset, works fine with Skype. There seems to be some issue, like a disconnect between incoming voice and outgoing. It will only work through the built in mike and speakers. I am on MAc OSX 10.5.8. Without the headset the other listener gets feedback of eharing their own voice, in the background. Thanks. Gene