I have a similar problem. Only more baffling.
I recorded my video, and it plays both microphone and game audio fine in a Win10 player however when I uploaded it to Youtube; the game audio is there but my commentary isn't present in the upload. I downloaded VLC and the microphone audio is there if I enable the option, but then that shuts off the game audio in its place.
So my question is more of a post-recording session. How do I merge the two audio tracks together so that they're playing at the same time?