Have you selected a different microphone than the one you want to use?
This is the most common problem when you're using headphones and your browser is confused about whether to use the built-in microphone or the microphone from your headset.
Here's the easiest way to fix this issue:
Click on settings.
Select "change microphone", which opens the drop-down button and then choose the microphone you want to use.
Save by clicking the "change" button.
You can redo your equipment checks and select the correct microphone from the start.