There are several voice changer ICs out there that might help. However, the closest that I see is the transpose function which will make the voice lower or higher. I don't think that is multi-level, though.
Another option would be to get a PCM encoder and decoder IC set. Put your arduino in between. You would then take a sample of a second or two of audio and spit it out to the decoder faster or slower depending on the effect you want.
Also, take a look at this:
http://elm-chan.org/works/vp/report.html