Under the “Speech” tab, adjust the selection found within the “System Voice” menu.

Open System Preferences from the  Apple menu and choose “Dictation & Speech”.

In earlier Mac OS X versions, changing the Mac system voice and speech rate is done here:

Adjust the voice selections found within the “System Voice” menu.

Select “Accessibility” then choose the “Speech” section.

Go to  Apple menu and open “System Preferences”.

If you want to change the default voice, you will find it is set in the “Dictation & Speech” control panel in modern Mac OS versions: This will use whatever the default voice is in Mac OS X, which leads to the next obvious question how do you change the voice used on a Mac? And how do you change the speed rate of spoken text on a Mac? How to Change Change the Voice & Rate of Speech on Mac Speech will continue until all words have been read aloud, or until the speech has been stopped by going to the same Speech menu and choosing “Stop Speaking”. Speech begins immediately through this method. Speech begins immediately, the Mac will use text to speech to speak the text that is shown on the screen or that is selected. Go to the Edit menu and then pull down to ‘Speech’ (or right-click and choose “Speech”).Set the cursor to where you’d like the text to be spoken (default will be the beginning of the document or text), or select a specific word or text.You can speak existing text or typing anything out to have it spoken too, here’s how text to speech works on Mac: Finally, we’ll also show you how to change the voices used, and the rate of speech (meaning, how fast the words are spoken). We’ll cover the two quickest and easiest ways to use Text to Speech on a Mac from common apps like word processors, web browsers, and text editors, and also demonstrate the command line ‘say’ trick to speak text by way of the Terminal application.

Mac text to speech voices online