Echo....Echo........Echo
Right click on the windows volume control icon and select open volume controls.
If Record control is showing, select options, select properties, select playback. (while there make sure that Microphone is checked so that you can see its control.
If play control is showing and the Microphone control is not visible then follow the steps above and make sure microphone is checked.
At this point you should see the Microphone sliders and a check box which says mute, check it and the microphone playback will be muted.