05-27-2018 03:35 PM - edited 05-27-2018 03:36 PM
I realize the Stereo Mix is missing while I am trying to use Google Doc voice type to transcript a video. Then I have found the solution here: http://www.randseq.org/2012/12/adding-stereo-mixer-to-windows-7-with.html
It is an old article, but it has worked on my x1 5th, HR. Hope this can be helpful for those who are looking for a solution for missing Stereo MIx.