In Southern parts of China and Taiwan, characters with an “eng” sound are commonly pronounced with the “ong” sound. In Northern parts you are more likely to hear an “eng” sound.
I will be sticking with these voices for now, since the benefits of hearing new voices seems to be offset by confusion over accents and clarity. The videos use a different voice for the female part, if that helps.
Thanks for this feedback though, since it really helps me plan the future direction of this course.