http://www.troublefixers.com/yahoo-messenger-voice-chat-problem-microphone-does-not-work-with-yahoo-messenger-call-setup-for-yahoo-messenger/
Yahoo call assistant setup, see where it says, microphone setup, Look for "Choose the microphone you want to use" and select the same device you have in windows.
To check windows,
Control panel, sound, Recording, microphone, properties and look just underneath "controller information". This will tell you the device name of your sound input card/port.