When you start your live video stream, the audio is disabled by default. To enable the live audio, click on the audio icon on the lower task bar, or click on the middle of your live view after pushing play the first time.
If you do not see this audio icon, your microphone setting may be disabled. To re-enable your microphone, go the the camera list. Click on the gear icon next to your camera. Enable the Microphone setting. When you return to your live video stream, the audio icon should now be present.