Before going on camera, consider using Audio Options to preview everything.
Click Audio Options at the center of your screen:
If someone is presenting during a Meeting (which hides the marquee), you can also access this at the top of your screen:
Here you can preview how you look on webcam and make adjustments to your background, sitting position or fix your hair before being seen.
As you can see you can also check your mic to ensure it is working or choose a different mic. As well as test your speakers for sound.
If you find your mic or speakers are not working, just click on DIAL-IN FOR AUDIO and join on the telephone.