MP3 Key Changer 

MP3 Key Changer is a small tool that has the ability to change the key of MP3 files. With this application, you will be able to transpose audio files, that is, change the key or pitch of any MP3 or WAV file that you have in your computer. It is also possible to speed up/down audio, in other words, adjust the tempo.

The application can change the pitch up to +/- 12 semitones, and adjust the tempo up to half or double the original (-50/200). The two operations are independent of each other; you can change the pitch without changing tempo, or vice versa, modify the tempo without changing the pitch. Any adjustment you make is instantly reflected in the playback.

The program has a straightforward, compact interface consisting of a single window, and very simple controls. It supports drag-and-drop, and you can also browse your PC to import audio files. Two playback options are available: low quality playback and HQ Preview, the latter letting you choose the audio quality. After you make all the desired modifications to your file, you can save it as WAV or MP3 with the possibility of choosing the bitrate.

MP3 Key Changer does a very good job and using it doesn't require any particular knowledge. The trial version has all the features enabled, the only limitation being that it only processes 1/3 of a song.