Excerpted from Mac OS X Mavericks: Peachpit Learning Series by Lynn Beighley
In many OS X applications that typically work with a lot of text, your Mac can read selected text to you. Depending on the application, you will find the Speech menu and the Start Speaking command in the Edit menu. Another way to access speech options is to Control-click (or right-click) selected text; then, from the Speech option, choose Start Speaking. If you want just a portion of the text read aloud, select that portion before you go to the Edit menu. The speech voice used is the one selected in the Dictation & Speech system preferences.