Any humanoid robot will have to start talking and you will definately want the lips to move in sync with the sound.
Its a great exercise to study the outside and the internals of any toy that talks especially the Halloween talking characters from the Gemmy company.
Then you can learn more about the technology used to put sound into your robot including the way it was done back in the 1990s and how it is done today with more advanced boards.
But start off by going to the instructables site and read about how people make moving and talking skulls. https://www.instructables.com/Automated-Talking-Skull/
The two companies that have been seling plastic affordable skulls are named Lindberg and Bucky.
Some great toys to look for that have speech and moving mouths are
- – Gemmy dancing Santa, Edwardian Butler, Mike Meyers
- – Chippy Ruxpin Bear (see the MakeZine youtube video by Andrew Langlety who makes it speak with an Arduino and a $9 C.H.I.P circuit) Here is how he did it. https://makezine.com/projects/chippy-ruxpin/
In the old days of the 1970s and after, talking toys and noise making toys had special circuit boards that could be printed with a few seconds of sound on them.
There was a Texas Instrument chipset that you could buy to put sound on for use in a toy.
Nowadays there are a few text to speech options onlinelike ttsmp3.com and in the cloud that you can use and of course there are products like Alexa that will talk.
Espeak is a Linux text to speech engine
CHIP is a $10 single board computer .
Check out the Arduino blog site by Peter Turczak where he takes apart a computer keyboard and adds an arduino and some wav files so that his child could press the keys and hear sounds. https://blog.arduino.cc/2021/01/18/old-keyboard-turned-into-a-new-childrens-learning-toy/