In the quest to play a voltage-controlled synthesizer with something other than a keyboard-like thingy (touch plates included), some have designed modules or other equipment that attempt to detect the pitch of an audio signal – say, from a guitar, flute, or singer – and convert that pitch to a corresponding voltage that can drive a VCO in unison with the original sound. This has proven difficult, as acoustic waveforms can be very complex, can change shape over time as a note dies away, and also may have unpitched noise components. These imperfections can cause false notes being detected, resulting in widely varying pitches being played by the connected oscillators.
There’s been reasonable success in creating guitar-specific pitch to voltage converters, but in other cases you must be very careful to play very cleanly (no overlapping notes, no bum notes, no extraneous noises) to get a generalized pitch to voltage module to work predictably. On the other hand, if you like crazy, related-yet-unpredictable results, go ahead and feed some audio into a pitch to voltage module and have some fun!