This part was a PITA. Basic idea was in some way to attach a servo to some part of the mechanism that drove the movement of the lips. Unhacked, when you hit the monkeys head, it pushs down a rod that switches the direction of the motor and engages gears that:

• Move the mouth
• Make the eyes move in and out
• Make a nasty screeching sounds by grating some spriny steel against a metal gear.

I tried several different linkages but in the end just kept cutting linkages until I finally found enaging a rod was the easiest way.

This pic is not that great but shows the head disengaged.

By going for the rod I had to remove the linkage that moved the eyes. I made up for that by replacing them with LEDs that I could then control with the LCD panel.

That's how I roll. make it up as I go. More fun, more challenge. No frustration of a plan not working as there never was one.