Why can't my Mac say "defeat ISIS"?
Speech Synthesis in OS X

As a user, you can affect macOS's text to speech pronunciation and approach. Everything you need to know is in Apple's Speech Synthesis in OS X.

You can provide TUNE Format information to control how the speech is generated. An Apple developer tool called Repeat After Me may also be useful in this endeavour.

Also see add pronunciation to text to speech.

Let Apple Know

However, you have encountered an interesting and serious bug in Apple's text to voice system. I recommend telling Apple's engineers directly either through the macOS feedback form or via the Bug Reporter service.

The problem can be quickly demonstrated with these two Terminal commands:

say -v Samantha "defeat ISIS"


…and…

say -v Alex "defeat ISIS"