How to speak on microphone only when key is pressed

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http://serverfault.com – I use Skype and Brosic to make conference video calls with my friends. Often there is a noise from the background. I use Centos and Ubuntu for workstation. Is there a way to configure Linux to activate the microphone only when I press a key for example Shift? I want people to hear me when I press Shift key. (HowTos)