Go to Main Navigation Bar > Meetings > select specific Meeting > Live Streaming (from Action Menu)

By default, the video stream will use the audio input of the encoder. (Typically, the microphone system).

However, you can easily mute/un-mute the audio, simply by clicking the Mute Audio checkbox from the Live Streaming Details fly out.

Note: Some splash screens force the audio to mute, depending on how they were setup.  In these cases, you will not be able to un-mute the audio as long as that splash screen is being displayed.