How do you exhibit this in Minecraft?

I just pressed some keys on my computer and now a voice reads out all chat letters. How can I issue this?

As far as I know, you can turn this off in the chat settings or the accessibility settings

It should be Ctrl + B

In the settings there's a point for it somewhere

Go to Settings → Music and Sounds.

Set the volume for voice / speech to 0%

First you go to the options on chat settings and turn off the voice output.