First, make sure your preferred recording device in Windows is set to the same device (soundcard) you are using for your Main ON-AIR output in OtsAV. To find and set the preferred recording device go to Start->Settings->Control Panel->Multimedia or Sounds and Audio Devices. (If you only have a single soundcard in your system then this will already be set correctly).
To switch Live Input on click on the icon on the toolbar. When Live Input is on any audio coming through your preferred recording device will be routed through OtsAV's audio pipeline.
To switch Live Input off click on the icon on the toolbar.
Note: Ctrl + Z or Ctrl + M will toggle Live Input on and off.
Live Input feature
How to set up a microphone through Live Input
How to set up an external device through Live Input