For some time now, there have been retail toys that speak or sing with mouths moving in synch.
I have a bluebird I bought at Big!Lots (Santa Maria’s current incarnation of Pic n’ Save) which tweets when you press one button and sings Zip-A-Dee-Doo-Dah when you press the other. I mean, it’s very simple opening and closing of the beak movement (there’s also a simple head turning movement) but the beak is synched up with the sound.
If this cheapie toy that’s decorating my bathroom counter can do it, why can’t the figures on Winnie-the-Pooh and Monster’s Inc.? (I feel it’s especially important on these two slow moving rides, as opposed to something like Mr. Toad, where you feel like you’re whizzing past.)
Furthermore, my stepfather has an extensive plush Tigger collection, including some that “bounce” and look much “bouncier” looking than the stiff, plastic-looking Tigger on the ride.
(Related tangent: I had an idea about re-doing the characters on Pooh to make them look less plastic-cartoony and more like toys come to life, which is what they were supposed to be in the first place – note the scene in “Winnie-the-Pooh and the Hunny Tree” when Pooh rips a seam while excercising, or references to Eeyore being "stuffed with sawdust.")
(Of course, as has been brought up on other threads, there are several older dark rides that do have characters with moving mouths.)