Beware the "mouth phonemes charts" you keep seeing. All the animation books show way too many mouth shapes that nobody ever makes in real life, like putting your buck teeth over your bottom lip for the "F" sound. Go ahead and watch people in real life or freeze frame a video. Nobody ever makes half of the shapes you're told about.
I like to stick to four basic mouth shapes. M (closed mouth) E (horizontal wide mouth) A (say ahhh like at the dentist) and OO (like you're whistling)
I'm also highly opposed to using tweening for mouth shapes, especially for stop-motion. It looks smooth and mushy and doesn't blend with the rest of the picture.