Since the introduction of Nuance speech technology in OS X Lion, which provides several new voice options in system preferences for the System Voice, many have compared the voice of Nuance-powered Siri on iPhone 4S to the improved text-to-speech included as free downloads within Lion. As noted by Reddit user Moosehadley, what you might not have realized is the downloadable “Samantha” voice for Lion is the same as Siri’s in the United States. Here is how to download it:
Open System Preferences> Speech> Text to Speech>System Voice> Customize> and select “Samantha” from the list. Apple will ask you to confirm the 469MB install.