It's not what you're after, but check out The Amazing Slower Downer. It can slow down or speed up an audio file, though its intent is really to let musicians slow down music to isolate the notes.
look into Audacity: http://audacity.sourceforge.net/
There's an mp3 export plugin available (on the site) that you'll need to download and install. It's free, and you can slow down or speed up the tempo