Have you selected a different microphone than the one you want to use?
This is most common problem when you're using headphones and your browser is confused on whether to use the built-in microphone or the microphone from your headset.
Here's the easiest way to fix this issue:
If that doesn't work the second time, try quitting out of your web browser and restarting. It's amazing how often this works.
You can also click on the small camera icon on the top right of the URL, click manage Chrome settings (chrome://settings/content/microphone) and select a different microphone.