> I'm not sure where that setting would be.
Doubleclick the little speaker symbol in the systray (next to the clock).
Check if there is a slider for the microphone, if there is, make sure the
mute checkbox is ticked. If there isn't one: archive menu, "properties",
select "playback" radio button in new window. Scroll through the list of
inputs and activate the ones you want to show - preferably at least
microphone...

, then go back to the main window and tick the mute box. Go
back to the archive menu, make sure "advanced options" or whatever it's
called is checked, then go to the properties window again. Select "record"
radio button this time and tick the boxes for the inputs you want to show.
Go back to main window. Now make sure the microphone is UNticked.

Also,
check the "advanced" button underneath, select the mic input boost box if
needed (you'll have to test yourself as it'll vary from mic to mic).
Good luck!