MicSwitch tool lets users mute or unmute their system microphone using a predefined system-wide hotkey. So, any program that uses a microphone can be stopped in an instant. Launch MicSwitch.

Click the playback device you want to mute or unmute volume for. Click on the volume icon to toggle mute or unmute per requirement. Right-click the Volume icon on the taskbar notification area ...

You can mute yourself in both the desktop and mobile Google Hangouts apps by tapping the microphone icon at the bottom of the screen.