Here are some checklists
✔ Software keys to turn on the microphone
In certain computers, there is a function key button that you need to press to turn the webcam on. For example, for Lenovo laptops, it will be "Fn" + "F4" key
✔ Hardware switches to turn on the microphone
Look around your laptop, sometimes there is a toggle switch to turn your webcam on/off
✔ Allow microphone use in browser
When you entered the classroom, did you "Allow" the use of the camera?
✔ Use the Chrome browser
Are you using the Google Chrome browser? It's the most reliable in getting your microphone working. If you don't, you can download it here
✔ Windows/Mac settings menu
Is Windows/Mac configured to use the correct microphone?
✔ Install a proper external microphone headset driver
If you're using an external microphone headset like a logitech, you may need to install the driver first.
✔ Plug the external microphone headset in and turn the power on
You can try plugging/unplugging the microphone, and find out of there are an on/off switches
✔ Quit other software that uses the microphone, such as Skype
Software such as Skype may get in the way of the browser being able to use the webcam.
✔ Refresh your browser page
Sometimes, reloading the page may help enable the microphone to work.
✔ Rebooting your computer
Rebooting a computer, especially if it's using Windows, sometimes can get a microphone to magically work.