Antares Kantos did this.
However, it was released around 2002, and appears to be no longer available from Antares, so I'm not sure it is possible to obtain a legitimate copy these days. A brief search suggests that there are sites which claim that it is freeware now, and possibly offer it for download, but I cannot vouch for those sites.
The general, brief sales pitch is/was:
Kantos analyzes any pitched monophonic incoming audio and
instantaneously extracts pitch, dynamics, harmonic content and formant
characteristics. This information is then used to control the kantos
From the review at http://www.soundonsound.com/sos/Dec02/articles/antareskantos.asp (emphasis added):
During my tests, I tried Kantos with male and female voices, guitar
and wooden flute as sources, and aside from detecting the wrong octave
for some of the female vocal notes, it performed pretty well. I was
particularly impressed by the way guitar and flute could be turned
into quite realistic-sounding bowed instruments — the articulation
feature definitely helps here — but don't make the mistake of thinking
you can throw any old audio at Kantos and then get great results
without making some adjustments. The input detector thresholds are
critical to good performance, as is the use of clean, monophonic
material, but as stated earlier, the chaotic results obtained when
feeding in drum loops can be quite interesting. I also liked some of
the effects that could be achieved by layering an untreated guitar
sound with the synthesized output for an unusual double-tracking
effect. Even the basic synth waveforms produce plenty of variety, so
experimenting with the couple of dozen or so included wavetables
widens the scope enormously, and if you get bored of those, there are
more on the installation CD, and more still on the Antares web site.
It looks like Native-Instrument's The Mouth does something along these lines, too:
Designed to generate melodies and harmonies out of whatever audio
material you feed it, THE MOUTH is a unique and exciting way to get
creative results on the fly.
Sing, beatbox or send a drum loop into it and THE MOUTH comes to life
as a tweakable synthesizer, bass, vocoder or talkbox-like effect. A
playful interface provides you with plenty of room to shape your
results as you go.