(excuse my english, i usually speak french...) This solution did not work for me at first time. Then I found that the driver
of my audio integrated card (ASUS ALC1200) was from Microsoft. I then looked to found the real driver for my integrated card (who was name before "High definition audio" and found the driver on the Web. Only after installing the good driver (after three times...), the Stereo Mix appeared (the name appear as Realtek High definition audio now). But I've still had to tried some input options in Audacity to get it works. But now it works great.