No audio input or output PowerBook G4

I just started using Skype and use a Plantronics USB headset/microphone. It works pretty well calling a phone but has a really annoying side tone when calling another computer.
My problem, though, is that since I started using the headset, just a few days ago, my built in microphone and speaker and the ability to plug in just a headset and listen have vanished. Also, the volume icon in the menu bar has disappeared. If I go to System preferences, there is only plantronics in the menu. I've tried restarting with no change. It used to work just fine...where might it have gone?

Thanks

I have to go to work now, but I'll try that this evening when I get back. I hadn't thought of that.
Thanks,
Rog

It's baaaack!

Rupert,
Don't know why or how...had to restart for another issue...tried to run disk permissions and it hung up so I stopped after a few...but somewhere along the line the speakers and microphone appeared in the preferences again. I do thank you for the help, mate.
Rog