@floid
Interesting links, thanks.
The problem with "mic" mixers is that they're always mono - microphones don't need stereo output, of course. The ST-MX3 looks very promising though, even if it doesn't take the usual audio jack or double coaxial. :-(
The 5.25" bay mixer on compgeeks looks very interesting - but it needs its host computer to be on, sort of defeating the point. I suppose I could always put it in an external HD case, but then where would I put the backplate? ;-)
I'm sure I'm not the only person who just wants a simple computer output mixer in a box with a minimum of features. But it seems nobody makes such a thing. :-(