You may be doing nothing wrong - could be a configuration issue.
Run the built-in Windows 10 troubleshooters. The troubleshooters may find and fix something.
Failing that then physically verify that the microphones and headsets are working correctly.
Start by swapping microphones, headsets, etc. with other known working microphones and headsets. Try your current microphone and headset on another known working computer. With and without Zoom.
Determine if the problem follows the microphone and earphones or stays with the computer.
Either way there is still more that you can do:
First, go to Zoom's website and search for Zoom audio options etc. that you, as host, can control on your system. Ensure that the settings are indeed as...