Hover your mouse near the bottom left of your screen when you are in a Zoom meeting. When you see the buttons in the image below, click on the upward arrow first and then click on the "Audio Settings…" option.

The Settings window should open to "Audio" page in the "Settings" window as shown below.

Under the microphone section, click and drag the volume bar to the left to lower the microphone's volume.

It is also recommended to un-check the option for "Automatically adjust volume".