Great stuff, @PopFiddle - I do a lot of play-alongs to existing work.  Well done.   And right here, right now - guess what I am downloading?   Thanks for the blender link - cool.  My basic video recording/editing software can't do anything like that - that would be super to use with someone else's video ( which at the moment, I have to strip the audio from, and then just mix with my own play-along, in Audacity etc.... ).

Thanks for the post !

@PopFiddle - I just select the audacity source as the stereo mix, play a you-tube video (well, with my broadband I have to let it completely load first to avoid hesitations) - and there you have it - here - I've just captured your audio from the vid - 

  So, Audacity now has the original performance.  I can save that as .mp3 ( or just play it, but I usually save it, and use Media Player to play the mp3 ) - and then I'll separately record against that, and then go back to Audacity and "line them up" and adjust levels etc.   My recording will usually also be a video+audio recording ( I use NCH VideoPad - but it doesn't allow the split-screen layouts you have done there ) - and I'll split MY audio from my video in VideoPad, take it in to Audacity, and mix the two.   I *could* then go back and place THAT combined audio with MY video - but - I rarely do that since it no longer attributes the original artist, and it looks just like my own work.   Here's a mix (it was just a fun thing, I was improvising, and "lost the plot" after about a minute in to it! ) to a video by a Harpist, Kora (on another forum) - so although she had posted a video, I left the combined mix as audio only - 

.... Anyway - that's the sort of thing I do - it would be nice to have better video editing software to allow video mixes / overlays.   I've had a quick look at the Blender - I didn't realise it was a full 3D imaging/design kit...  wow....

I'm in Audacity, and select from the menu option - e.g. -

  .. that seems to do it for me.   I would "kind of assume" this would apply across different systems, regardless of the sound "card" ( mine's not a card as such, just the inbuilt RealTek audio chipset in my laptop )

@popfiddle - great - glad that helped!