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.
(I have a MacBook Air with a built-in mic, with OS X Yosemite.)