Yea I would have to agree, from the look of the movement I would guess pneumatic too. It is to sharp of a movement to probably be a servo. I am not sure how you would get a cylinder to mock that movement though. It could be some kind of motor though since that looks professional not homemade. The left right movement is probably servo though.
It does not appear the the mouth moves that much, so you could use a solenoid. It will be quick of course. I have machines that have 24 volt solenoids that pull about 1/2 inch. Just pulse when you want mouth to open. Oh and solenoid are plenty strong. The ones on the machines pull down spring loaded rollers. The applied pressure on the rollers exceeds 20 lbs of force.
Use the wiper motor or maybe something smaller with a simple and short stroke. At the top of each stroke the mouth gets bumped open.
Have the plants speech on a cd and play continuously. Use amplified speakers to play the sound.
Use a switch or 110 v remote (Home Depot) and power the motor and the amplified speakers from this. Keep the cd player running constantly with a seperate power source so there is no "lag" time waiting for it to spin up.