When you join the greenroom for the first time you will be asked to select a mic, but you can change that at any time before or during the broadcast.
Changing your mic
Click the guest icon under the Effi logo.
This will show you your video feed. Hover to see more options.
Click the quick actions menu and choose "Video & audio."
This will show you all your available video and audio options. Click the "Microphone" dropdown to change your mic.
When you are satisfied with your options, just click the "X" to leave and get back to your event. Change your mic and any other options as often as you need to!