I'm not sure what you mean. In your Duolingo account settings you can say whether or not to use the microphone, but the actual control of the microphone rests with your computer's operating system, probably with the settings for your sound card. Not sure if this helps...
I know what you mean, it sounds all scratchy on this site. I have the same problem at livemocha (another language site). with this one, I can still usually get it right, if I wait two seconds before pressing the stop record button
What operating system are you using? On Win XP or newer find "Control Panel". Find and click on "Sound" Click on the "Recording" tab. Test which ever one you want (head set, external mic etc) and choose one as your default.