Mp3 Speed is a free software which helps you in changing the speed of the MP3 files which is useful when you want to hear an audio slowly to get a clear understanding of it. Apart from speed it can also change the tempo and pitch of the audio.
As you can change the tempo and pitch you can create some of your own mix and that without installing any codecs as the application comes along with some already. It supports converting multiple mp3 files in one go.
Few other features worth mentioning are removal of dead mp3 files and duplicate files.
The converted files can be saved into a separate directory and you can very well control the bit rate of the audio file. The only thing missing here is a preview of how files will sound after getting converted. So if you have a huge MP3 file, I would suggest you to use an MP3 Cutter, get a preview and then convert the complete file. Download MP3 Speed.
Related : How to easily batch convert MP4 to Mp3