Why no stereo mix available on Kmix using Realtek HD audio
 

I have just been brave enough to make the switch to Kubuntu on my desktop PC, I might do it on the laptop too but that's for another day.
I am using an Acer Aspire L100 with on board Realtek HD audio.
Using the previous OS I could record the "stereo mix" output from the card.
I do not seem able to do this with the ALSA sound system in operation.
I have tried various applications such as Audacity, KRec and others.
None seem able to record the output apart from one combination in Audacity that (just about) managed to record one channel extremely quietly with masses of hiss. This may have been a "Bleed through" in the electronics.
I have googled around and tried to install vsound which I couldn't work out how to do.
Is there some way of enabling the "stereo mix" input that I'm missing?
Also what is OSS that Audacity gives as an option?
Any help would be gratefully received.