The reason tin whistles will work well on automated processes and voice doesn't is because the whistle is a tuned pipe and will play the same note each time (or it is obvious if it doesn't). The human voice has the psychological factor that none of believe we sing badly or embellish as much as we do (well most of us anyway) and our larynxes are not tuned pipes - it is our brain via ears that are tuned, and not all of us have perfect pitch. Those that do often acquired it by practice. Those that can only sing are rarely trained. I speak from personal experience. My first attempts were with a guitar tuner and it latched onto harmonics. My DOS program got better the more I used it!!!!
Isn't the brain a wonderful thing ?

My Windows prog has just passed a major hurdle but it relies on free "FFT API's" for speed and the instructions were barely adequate in Chinese, in English they are sub-basic. It is still a few months away.