Quote Originally Posted by Evil G
the way we recognize that an "ah" is the same sound, no matter what note it is sung at, or an "oh" is different from an "ah" even when sung at the same note, is by the next couple of harmonics above the base frequency. these combinations of harmonics are called "formants" and a "formant filter" will make any synth or drum sound like a human voice by highlighting the required odd and even harmonics above the base note.

i don't have a chart handy that lists which harmonics are required for the different vowel sounds, unfortunately, but the info is out there.
I got one somewhere... I'll try and dig it out