1. Enable Dolby Pro Logic

Ensure 'Dolby Pro Logic' is selected in System Preferences --> Perian under Audio Options.

2. Connect your receiver.

Audio connection must be a digital connection (optical or HDMI).

3. Change sample rate to 48KHz.

1. Open the 'Audio MIDI Setup' app located in the '/Applications/Utilities' folder.
2. Source should be set to 'Digital Out' 
3. Change the format to 48000.0 Hz, and 2ch-16bit

4. Check receiver settings.

Some receivers have an encoding setting. (example: some Sony receivers need to be changed from DEC PCM to DEC AUTO)

5. Check Front Row settings

Enable AC3 Passthrough in Front Row via Media --> Settings menu.

6. Enjoy full 5.1 surround sound!