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Have messages read out loud in Messages on Mac
Messages can speak the text in a transcript, the Messages window or a profile window.
In the Messages app
on your Mac, select a conversation or select text to be read.Choose Edit > Speech > Start Speaking.
To make the speaking stop, choose Edit > Speech > Stop Speaking.
To change the computer’s speaking voice, choose Apple menu 
in the sidebar, then click Spoken Content on the right. (You may need to scroll down.) Click the Help button 
for more information.