1. Have you clicked the microphone button to share? Are you muted?
2. Are your microphone's settings correct? Have you selected the correct microphone to share?
3. Is there another program that may be using your microphone? Like Skype or Hangouts? Quit those programs and restart your conference room.
4. Restart your web browser or switch browsers and/or restart your computer.
5. If you are using a headset, make sure to plug in the headset before you enter.