Before going on camera, consider using Audio & Video Options to preview everything.

Click the Settings (Gear) icon on the upper right:

Under Audio & Video Options you can preview how you look on webcam and make adjustments to your background or sitting position. You can also see your Dial-in information if on phone, or check your mic to ensure it is working or choose a different mic and test your speakers for sound.

The "Operating System Settings" option in the device list will be selected for all new users by default but if you choose a specific device from the list, the app will remember your preferences and will use the device you chose next time:

Note: when the "Operating System Settings" option is selected in the device list and you change the audio devices in your OS settings, the meeting audio device will immediately be changed to the one the OS is using at the moment.

As for other options, you may also do a quick check on your Webcam using the corresponding icon at the bottom of the screen:

Or test your speakers, check your mic or switch to telephone using the corresponding icon also at the bottom of the screen: