Listening to audio input in Vista?

This is driving me nuts!

In XP I used to be able to plug in a microphone, unmute it as an input device in the sound mixer, and then I could hear it live through my speakers. I can't find the same option in Vista, in fact the mixer does programs not devices...

The audio's coming from an onboard Sigmatel chip - "SigmaTel High Definition Audio CODEC"

not sure if this is what you're looking for but it may work

Go to control panel (classic view) - sound - recording tab - right click in the white area - select 'show disabled devices' - right click on stereo mix - select 'enable'