I'm using Windows XP. I have plugged in a microphone, and I need to send the input immediately to the headphone for all the time the microphone remains active. How can I do this?
Open the volume control. From the menu, chose "Select devices for playback" (or something similar, I'm not near Win XP right now). Check the microphone check box. From now on, the controls for your microphone should show up on the volume controls. Uncheck "Mute".
This works only if your sound card supports it (which should, but all sound cards are different, so I don't know).