I use Rubberband quite a lot. It's a quick, easy way to change keys and tempo. It saves files in the .wav format, but I can change that to mp3 easily in iTunes.
Keep in mind that, with any app that does what Rubberband does, , if you slow the tempo too much, the audio is going to get thin and "warbley", but since I'm only changing pitch and tempo parameters for music practicing purposes, that isn't such a problem. Like when you stretch an actual rubber band too far, it gets much thinner.
With this app, you can fine tune the pitch up or down to match the exact tuning of your instrument, as sometimes recordings are not precisely tuned to standard frequencies, and are just a fraction above or below a true pitch. For convenience sake, it's paid for itself many times over.