A free program for Android, by Lyrebird Studio.

AudioApp is an awesome app that should make your phone personalization much, much more authentically you. It lets you create a custom ringtone out of any audio file you want and the best thing is that you can create a ringtone for each contact so you don’t have to remember a specific number.

It has a very simple interface and a lot of great options. First of all, you need to select the audio file you want to modify. Once you do that, you will have the option to modify the pitch, the volume, and speed. You can even change the voice altogether.