Text-to-Speech (TTS) is a feature that allows for your text to be converted into a computer generated voice. Text-To-Speech is automatically enabled for your messages, but if you would like to disable the TTS feature, all you need to do is uncheck the box for Use Text-to-Speech for Voice Message on the Compose Message page of your alert.
Tip: Spelling and punctuation count! Make sure that your message reads exactly the way you want it to sound.