Changing key on an MP3 - ?

I'm a private music instructor - a mother of an ex-student called and asked a Q I could not answer - she records music and is collaborating with another musician. From time to time there's a background track (likely MP3) that they'd like to change keys on. She's wondering if there's Mac software that can do that. I'm familiar with notation software, but not knowledgeable in this arena!

Appreciate any advice!

I've used Audacity as a free app, but prefer Amadeus now with a small price.

Amazing Slow Downer - the musician's friend.

This is a great for learning new pieces - can slow down a song up to 20% of original or increase the tempo up to double without changing the pitch.

Can also change the pitch by semi-tones.

He has versions for Windows, OS X, and even iPhone or iPod Touch.

All real time - can't re-record the track in a new pitch or speed with this software only.