Garage Band is an excellent tool for this.
You can import songs, change the key and even slow them down. Even better, you can write your own chords in musical notation and create your own backing tracks, even play them in loops. There's a learning curve but apple ilife has excellent on line help and it's pretty intuitive. Good luck with it. Feel free to reply with questions. JB.
"Thanks Mike. I have to prove to myself that I can play the material as written but once I've done that I start to allow myself some leeway to have fun and drop in my own bits. There's not too much space to do that in SK's version of…"