While in Safari, TextEdit, Pages, and other applications; select the text you want to be read, then click the application name in the menu bar. Go to Services > Speech > Start Speaking Text. Your Mac will then use the default voice to read the text. It will continue reading until it reaches the end of the selected text, or you can select Services > Speech > Stop Speaking to end it immediately.
You can change the default voice by opening System Preferences (Apple menu > System Preferences) and going to Speech > Text to Speech. Once there, select a voice from the drop-down "System Voice" menu.
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