Go to the bottom left corner of the zoom window.
Make sure that the microphone icon is not crossed (which would mean that your mic is muted).
If this does not solve the problem click on Audio. A window will pop up. If you see a green box saying "Join Audio Conference with Mic & Speakers" click on it !
If the issue is still not resolved then in the same pop-up window click "Test Mic & Speakers". If the test is unsuccessful make sure your mic & speakers are plugged-in correctly and are working properly.