Matt, have you heard the term "freqwee" ??? I read long ago that this was the device used by radio personalities to alter their voices.

The method I remember (analog) involves balanced modulation of the original by a high frueqency. the A+B result is filtered out. The result is again modulated with a higher frequency. The resulting A+B is again filtered out, leaving a pitch-shifted copy of the original input. I can't find this anywhere on the web, and I have too many (and thrown out too many) magazines to find it.

This also may require too much processing for an 8051 (think you can do it duh ???), but it is a "doable" analog method.