And just to be extra confusing ...
... generally, when you plug a PC's speaker (headset, whatever) into the microphone jack, it can be used as a microphone. Thus bringing about "but they can hear me, but I can't hear them!" confusion.
I recommend that you do NOT mention the hows and whys of this to your more ... excitable ... users. Just fix it and walk away like a cat, tail up, without looking back or saying anything. Trust me, it's easier that way.
Biting the hand that feeds IT
