How to stop the 'live' mic sound playing through the speakers?

I assume you mean that the microphone sound is being played back through the speakers constantly.

Try the following:

Go to the Control Panel, and click on Sounds and Audio Devices.
Under Device volume, click on Advanced.
Under Microphone, check the Mute button.

If the "Microphone" section is missing, go to Options -> Properties, and under the Playback section, enable it.