You will get a permission popup if it is the first time you are accessing your microphone. If you have skipped or disabled it by mistake, you can click on the disabled icon on the top right corner of the chrome search bar and enable access again. Or you can always go to browser settings and make sure you grant access to your microphone for the particular site. For Mac users on the latest OS, check if you have given permission for chrome to use the mic in the system preferences.

Yes, you can use an external microphone that supports your browser. Make sure the proper microphone is selected

As your first step, check if your browser has the permission to access mic. For Mac, In System preferences > Under Security & Privacy > Microphone, you will have to check if you’ve granted permission for your browser(Chrome/ Firefox) to access the microphone. If the permission checkbox is granted and the issue still prevalant, try restarting your system or your browser.

Sometimes a simple browser restart can do the trick. Or go for system restart.