Hi,
The PIC processor alone won't be able to do the job because it requires a lot of math processing power to completely change the nature incoming audio signals i.e change its pitch, base frequency, tone etc. Fot this you may require a DSP (digital signal processor) which is very fast at doing these conversions.
Or if you only want to add noise (Robot like) to the sound, it can be easily done with the PIC. Just chop the digitized audio signal with a particular frequency. But I strongly recommend DSP for all type of audio processing.