How to customize new Siri voice in the Speech of Accessibility?
The answer is simple--but I think you may not like it. Siri's voice can be selected for text-to-speech only on iOS devices. The reason you do not see it listed among your MacBook's System Voice choices is that Siri's voice is not available as a System Voice on MacOS. On Apple's computers, Siri will be heard only in its role as the Siri personal assistant. You need to use a iPhone or an iPad to have Siri speak selected text.