Step-by-step
Select the three dots in the bottom right corner of Google Meet (you might need to hover your cursor near the bottom of the window)
Select Settings
Under the Audio tab, check to see what Microphone is selected. If you have recently connected a Bluetooth device, your Chromebook might be trying to use the device’s microphone.
Select Internal from the drop down menu beside Microphone
Select Done
More Information | Google Support Site