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.
Option 2:
You can redo your equipment checks and select the correct microphone from the start.