1. Make sure you have the right to share your microphone. Ask the presenter in the chat.
2. Have you clicked the microphone button to share? Are you muted?
3. Are your microphone's settings correct? Have you selected the correct microphone to share?
4. Is there another program that may be using your microphone? Like Skype or Hangouts? Close those programs and restart your conference room.
5. If all else fails, restart your web browser and then your computer.