How do I route YouTube audio to AU Lab using SoundFlower?
Crank the computer's volume all the way up. (This is so that you pass 100% of the sound into AU Lab).
In System Preferences => Sound => Output tab, select Soundflower as output.
In an AU Lab document, select the same Soundflower option as your input. You should then be able to see Youtube's audio levels in AU Lab.  


Source.