You can change the setting at either Session view or Edit view.
Go to Options > Windows Recording Mixer
You will see a small window that says "Volume Control"
Go to Options > Properties
Select "Recording" then click "Ok"
You will see a mixer with many faders and controls for different type of recordings. Deselect all of them EXCEPT microphone (cos you only wanna record audio from your microphone, yes?)
Hope that helps.