I've had this recurring problem where my laptop (running Windows 7) refuses to recognize and external microphone being plugged in. I've tried numerous external ones (in case the mic was broke), and still the same result. I wouldn't have a problem, except that my laptop's built-in microphone picks up a lot of background noise - it sounds like there's a tempest in the room!
I've tried Configuring my microphone, but, seeing as it only recognizes the built in one, I've had no luck with that.
you need to look at the motherboard drivers for the audio, from the computer manufacturer, the motherboard manufacturer, or from the manufacturer of the hardware used by the computer maker... download the latest update for the audio,
check the manufacturers for bios update bulletins which might be designed to solve such problems, check the manufacturer's manual for audio connections...
by clicking the speaker icon in the lower right of the desktop this should bring up the audio mixer settings.