Vocal harmonizer

This is hard, as witnessed by the way
the cheap/free ones sound, and the expense
of the better ones like Harmony4 and Anteres.

Not sure how much progress you'd make on 
this, but your ideas on the sibilance
pass-band, combined with a simple zero-crossing
or other based consonant detector, are
definitely in the right direction.

Shifting formants up or down actually isn't
as easy as you'd hope, but it's possible to
flatten a spectrum, shift, then reapply the
spectral shape after, yielding a better sounding
pitch shift.

Check out the PitShift UG in ChucK.  It's
not pretty, but it's cheap, and could be
improved, used, or worked around to make it
better.

Forge Ahead!!