Mix mic audio and system audio and simultaneously record using Audacity I want to play an instrumental track that a friend uploaded to YouTube and simultaneously sing into the Mac's mic. And I want to record the mix of these two sounds using Audacity.

I've tried using various solutions for similar (but not exact) situations that are described on Ask Different. I've tried various combinations of "Aggregate Device" in "Audio Midi Setup." Nothing has worked so far.

I have the following installed on my computer: Line In, WavTap, SoundFlower. 

Thoughts?

(I have a MacBook Air with a built-in mic, with OS X Yosemite.)
I'd approach the problem from the opposite end...  

Get the YouTube audio track into Audacity first.  

You can use iTunes to extract audio from video  


In prefs > General, click Import Settings... &amp; change 'Import Using' to WAV [remember to set it back to your preferred audio format afterwards] 
Drag your video into iTunes, then go to the Movies tab  
Select the movie, then File menu > Create New version. Hold the  Alt ⌥  key &amp; the 3rd option will ungrey, allowing you to save the audio separately.
Import this to Audacity.


Then you have all day to record your vocal track separately; add reverb etc &amp; export the finished mix from Audacity.

This approach not only gives you the  time to go back &amp; edit mistakes etc in the vocal track, it also avoids the essential difficulty of trying to live-record 2 audio streams to separate tracks without professional audio equipment.