Starting with OS X Lion there are now many more regional voices available in OS X. The US and Fun voices are installed by default, however the additional voices require the steps below to install them.
1. From the Apple menu, open System Preferences, then choose Speech.
2. From the Speech preferences panel, select the second tab: Text to Speech.
3. Open the Voices drop-down menu, and select Customize.
4. Choose the voices you want from the options available, and click OK. You'll be prompted to install them, click OK.
5. Talk To Me will use the voice set in the application. If you choose DEFAULT in the application, it will use the voice set here. If the voice is not an option in the application, then set it here in System Preferences, and use the DEFAULT setting in the app.