Hi, I'm David an independent advisor and user just like you. I'll try and help you solve this problem.
You may have accidentally set the option called "Listen to this device" in your microphone properties - when that's enabled whatever that input hears is relayed through your speakers.
To check this first open sound settings: right-click your speaker symbol which should be at the bottom-right of the screen and choose "Open Sound Settings".
From there you should have an option in the right-hand side of the window that appears called "Sound Control Panel" - click on that.
Another window will open listing your sound devices - click on the one called "Microphone" and then click the "Properties" button, from the next window click on the "Listen" tab... you may then see a checkbox labeled "Listen to this device" if that is ticked clear the tick by clicking on it then finally click OK.
Regards,
David