In older versions of Mumble, Text-To-Speech was turned on by default. Thankfully newer versions now ask if you want to turn it on during the audio wizard.
If you've accidentally turned on Text-To-Speech, and want to turn it off, here's what you do:
- You do not need to be connected to a server to do this. Simply launch Mumble, cancel the server browser, then click on Configure → Text-To-Speech.(screenshot)