Why can’t I speak?

I've long done something I call "finding your mouth" in a lucid dream. I have a lot of speaking issues in dreams, even still, likely because it's a reflection of the person I am in waking life. I've always been quite shy and, even when working past that, I'm still a soft speaker. In my case, I've come to accept that my dream is simply modeling the waking self as best it can, leading to some difficulty if I try to speak before preparing. Before speaking, try to exercise your dream mouth a bit. I typically lick my lips, open and close my mouth a few times, and hum and clear my throat before speaking. I find that this helps, and kind of solidifies the idea that my mouth is there before I try to speak. It's like working up to speaking and, with each successful step in finding my mouth, I find that it builds the expectation—within that dream— that my mouth is there and is capable of speaking. Kind of an odd thing, but something that has worked for me for years.

This isn't something that would be expected (I've certainly never experienced this in decades of lucid dreaming), I think it's most likely that this happened by chance in one dream, and as a result you've built yourself a negative expectation which becomes a self-fulfilling prophecy.

Try to drop that expectation if you can, as it's not based in any limitations of the dream world.

Also, remember, talking is just a convention of the waking world where we are limited by pesky physics. In dreams you can just "think really loudly" and communicate telepathically with your dream characters.

So even if this issue persists for a while, just think what it is you want to say to your characters and they'll respond accordingly. You'll most likely find that after a few times of doing this that you'll regain your powers of speech.