In most cases, you won't need to think about which microphone you're using. In the event you have an external microphone attached to your computer, you may need to test it to ensure it's selected and working as needed.

Next to the Begin encounter button, you'll see a quick settings menu . If you click the button, then you'll an option to select your microphone. Next to the microphone picker, you'll see a meter that shows the input volume of your microphone.

In Settings > General, you'll find a Microphone option. Here, you can select your microphone and test the levels.