go to the Now Playing button on the top of the window and right click on the arrow under "Now Playing" and a drop down box will pop up w/ options and there should be one that says "Enhancements..click on it and more options will pop up then click on Play Speed Settings
I've been using a plug in for Winamp for years now, called "Take it easy".
That works great! Since I always tune down a half step, I can just tune the mp3 along with me, or for example, when the song is in Drop C, I just tune the mp3 up a half step, flip the Hipshot detuner on my bass and I'm all set.
It wobbles a bit here and there, and singers usually sound really strange when tuned up or down more than a full step but you don't hear that when playing along.