Why did Egon become a mute?

LARYNGITIS...SAME SHIT HAPPENED TO CAROL BRADY.

I don't remember any of the other ghost in the movie talking.

Winston was the one in Ghostbusters 2 that said, "It's always the quiet ones." Also Harold Ramis has been dead quite a few years which is why Egon didn't talk.

because the writer director is a fool and his idea for this movie should never have been greenlighted

The length of time he appears at the end with his smile and nods and not saying anything, or mouthing anything at least, did feel a bit odd. But he made up for the problems he caused by being right in uncovering what was going on and helping to save the world again.

It is not often we see someone who acted in a franchise, who has passed away for real, being recreated as a ghost version of the character in a movie, so it's a bit of an unusual situation for sure. I wish they could have hired someone to speak just a few simple but powerful words in his voice (even if AI could somehow do it), but it may have been a bit disrespectful or something. I'm not sure if that's the right word. In the end, I'm glad they left it as a visual thing only. Having a spoken voice may have been too much, even it was just a "shhhhh" to calm his emotional family like the library ghost at the start of the first movie.

Were ghosts, who not fully got a body (shiny aura) ever able in Ghostbusters movies able to speak? In the cartoons yes, but as I am aware, the shiny ghost forms never speak, only shriek or are silent. The ghosts able to form a normal body can speak, but they are not shiny ghosts. So I assume its one of the aspects of the ghost classes. Gozer and Ivo Shandor are then Class 7 according to the movies. Egon was definitely not yet Class 7.

The first reason is obvious. Harold Ramis was dead in real life and couldn't provide an authentic-sounding voice.

Second, nobody could ever truly imitate Harold Ramis's voice properly, and fans of the franchise would have spotted the different voice in a second.

Third, AI was only starting to be used in the film industry when this movie was being made, and if the tech evolution for that is going along the same lines as CGI, any use of it would have been extremely expensive, and like all films, this one had a budget to stay within. Chances are that the voice version of AI hadn't been perfected yet (and still isn't completely up to snuff as of this writing) so they didn't have the ability to use it just yet.

Fourth, they would have had to ask Ramis's family permission to create lines for him, even if AI audio was available.