[AChimp]

When you talk into your microphone can you hear the sound coming out of your speakers?

That's how mine works. Your microphone is always on when it's plugged in. Any sound it picks up will be echoed in the speakers to show that it was received (it talks about this in one of the system configuration windows somewhere). You can mute your microphone when you're not using it using the volume control window.

If that doesn't fix the problem, you somehow turned on a weird sound effect.