I’ve been going through the FM8 manual, and this description entertained me:

Mouth controls the Talk Wah frequency. Turning counter-clockwise gives an “oooo” sound; moving it toward center morphs into more of an “aah” sound, while going further to the right morphs into more of an “eee” sound.

Size adjusts the size of the “virtual mouth.” When counterclockwise, the mouth is small, like that of a midget. When clockwise, it’s like a giant’s mouth.

Thanks for the colorful description, NI!