Headphone/microphone troubleshooting bleg.

I’ve got one that plugs into analog jacks on the computer – one for the microphone, the other for the speaker.  The speaker jack works fine: I can hear the sounds come through, no problem. The problem is the microphone: Skype doesn’t see it, Mumble doesn’t see it, Audible doesn’t see it, Windows 8 itself doesn’t see it.  How do I convince the computer to recognize that there’s a microphone there?