I use Audacity for my own practice, and I recommend it
to my students
(they're all on Windows, Audacity is cross-platform). The Change Tempo
and Change Pitch functions are wonderful, I use them frequently during
lessons here.
http://audacity.sourceforge.net/
It's in most distros these days, so check your repo for an installable
package.
Best,
dp
Be aware that some repos compile Audacity without mp3 support, so you'll
need to build your own if you need to import mp3 files.