Hi, I've tried all this and I'm still having problems ...
I have a Dell XPS M1530, running Vista. I uninstalled the SigmaTel codec and installed the older XP driver. It's working fine.
HOWEVER, Stereo Mix still will not show up in the sound devices list. (I have the disabled/disconnected boxes ticked.)
I'm really frustrated, I've tried everything I've been able to find online. I hope someone can help. Thanks.