Use this:
https://youtube-dl.org/
Might take a bit of setting up but once it's running it's the easiest thing I've found. AudioHijack is realtime whereas this is super fast.
Once installed, the following terminal command will extract the audio from the youtube link in wav format.
-x means extract, so you just get audio, no video. As far as I can tell from googling, youtube-dl automatically selects the highest quality encoding available when downloading.
youtube-dl -x --audio-format wav 'https://www.youtube.com/watch?v=Y73kxcubDBE'