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).
Set up your Live Input device in OtsAV. For instructions on how to set up an microphone click here. For instructions on how to set up an external device click here.
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.
How to set up a microphone through Live Input
How to set up an external device through Live Input