For the best audio you should NOT use the built-in microphone on your streaming computer because the built-in computer microphone captures sound at the streaming computer whereas the goal is to capture the sound near the point of origin such as the podium microphone. Audio should be brought in electronically via the "Line In" (or microphone input plugin) on your streaming computer. This can typically be done by running an audio line from your audio mixing board or camera directly into the computer's microphone input port (typically a 1/8" input plug).  

If bringing the audio in through the computer's microphone input port doesn't work you may need to utilize an audio capture device (or some video capture devices, such as the Blackmagic Design Intensity video capture devices, also allow for audio capture).