Help with Festival voice's

Alright, anyone know how to edit the voice file or know of any links.
Currently, I’m using festival on asterisk and have tried more than ten different voices ranging from cmu to nitech. However, the voice still sounds very robotic and jittery and unsuitable for use with ivr.

Anyone know how to edit the any config files so that the voice sounds more pleasant or smooth.

Any help would be appreciated as i’m new with these two programs. Thanks