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.To search for contacts and content in conversations, click in the Search field in the top-left corner and enter what you’re looking for. Or choose from the suggested contacts, links, photos and more.
If you’re restricted to sending and receiving messages with only certain people, an hourglass icon
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 
for more information.