Changing tempo of a mp3.

I am trying to remix a pre-recorded song with gargeband....the original tempo of the song is 91 bpm. i imported the song into gargeband (as a 91 bpm project and a seperate time as a 120 bpm project) and both times i have been unable to speed up the tempo of the song. I even tried saving the song as an apple loop and opening that up and trying to change the tempo of it. All of the other apple loops (pre-installed MIDI ones) were able to be slowed or sped up but the audio track remained the same. How do I change the tempo of a 91 bpm song to a 120 bpm song? I realize the pitch will be higher (more chipmunk) but that can be fixed with the pitch feature. If this is possible with garageband please let me know. Thanks.