(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.