Is there a sound icon on your side bar?
What to do if there is no sound icon
If not, go to Start, Settings, Control Panel, or Start, Control panel depending how you have Windows set up.
Click on Sounds & Devices.
Make sure "Place Icon In Task Bar" is ticked.
Now, press Okay, and on the side bar, the sound icon should now be there.
Double click it, and you'll get a set of volume bars, and if the microphone is noted there, simply mute it when you don't want it by clicking the "Mute" button.
How to get the microphone to display in Play Control
If it's not listed, go to Options, Properties, and in the list, tick Microphone, and press Okay.
now it should be on the list, and you should be able to mute it from there.
I hope this helps