VLC media player (free) will happily download youtube videos into many different formats - video and/or audio.
 
And does it a lot faster than if you played the video from start to finish.
 
 

Freemake Video Downloader will download video and convert to several different formats of which one is mp3. I use it or VLC, whichever is at hand.

Yeah... all those are probably easier options.
 
The easiest likely being just downloading the vid using one of those YouTube download apps. Then you can just import the vid into Sonar and the audio will get stripped out into a new audio track... where you can do whatever you want with it.
 
I however usually just hook the headphone output of my laptop (a separate computer from my DAW) to my mixer using a Y splitter, send the direct outs of those channels into my interface (which are always hooked up via a snake anyway) and record the track in real time. I can use the mixer to control levels and add some EQ if I want as well.
 
I don't do that often though but it works nicely.
 
Now that I own a half decent screen cap software I can actually just record the vid (or just the audio using a special feature) into the screencap recorder and then do whatever I want with the audio after.
 
So... definitely lots of ways to do this.
 
I just saw the Voicemeeter thing mentioned in your OP and since I've been messing with it lately figured I'd point out how you could do that.
 
Cheers.

+1 for vlc

 
Note on VLC :
 
For some reason, my VLC only works properly if I have "Tools/Messages/Verbosity" set to 1 or higher.
 

I have Virtual Audio Cable (VAC) which I think is a similar kind of thing. I use it to route all of my system audio through Pedalboard2 on which I have the ARC2 plugin installed, that way I can room correct all of my computer's audio output with ARC2. To be honest it took me a lot of trial and error to get it working. A lot of head scratching as I wondered which input/output options to select to get the audio where I wanted it. And VAC has a lot of very technical options too. In the end I stumbled blindly upon the right options and quickly saved them. 

Sorry 4 late reply, been away...

I Also use Audacity, and that works, just seemed easy to record directly into Sonar. Well, haven't solved it with VB audio so will continue with Audacity for now..