Yes, Magic AB works fine with Sonar X3.
I knew I had wanted to get it, but when I moved up to Sonar X3 and recognized I would want to do some things with a fresh approach I decided to get Magic AB at the same time I moved to X3. I've been using it and it's great. I've found it does not take mp4 songs straight from my iTunes music folder, but it takes mp3s fine and I have plenty of those. I can convert mp4s to mp3s if I need them for reference purposes.
As a side note, I also use the Focusrite VRM box, and the combo of having reference tracks easily comparable, and being able to toggle through different speaker types on an A B basis between a song I'm working on in X3 and reference tracks is both pretty neat and pretty convenient. It's pretty cool. And Sonar X3 is fantastic!