Audacity (http://audacity.sourceforge.net/) is an open-source application for recording and editing digital audio files.

One useful feature of Audacity is to manipulate the playback speed of an audio file. One can use this to slow down a song in order to learn it (such as what I do for my daughters' violin lessons) or to speed up an audiobook. To do this, open up the MP3 file and select the Change Tempo… option under the Effect menu and set a desired percent change. This will not change the pitch of the music. After you are happy with the setting, you can use the Export… option under the File menu to save the new file.