Thanks to both of you! Now the "Stereo Mix" makes sense.
I've tried using the mic jack as stereo and it only seems to see the left channel, with it fed over to the right channel as well. My mini has a cracked screen, maybe when it gets exchanged, the mic jack will be stereo. I tried "Audacity" with the mic setting to two channels, and also GoldWave. There could be some sort of short in the jack or on the printed circuit board having to do with the Left and Right mic channels.