I used to have my home theater receiver hooked up to my Mac Pro via the optical connection, and for a long time, enjoyed listening to my music in an emulated surround mode through Dolby Pro Logic on the receiver. Since moving into a new apartment, however, I had been using normal stereo mode.
When I finally switched back to my 5.1 setup, I found I could no longer have my music mixed to five channels! For whatever reason, Leopard was outputting my audio at 96 KHz, which was above what my receiver could process. In order to fix this, use the Audio MIDI Setup application found in the Applications » Utilities folder. Use the Properties For drop-down to choose Optical Out, and then down in the Audio Output section, choose 48000Hz for a format. Restart any running applications using audio and play with your receiver; you should be able to use Pro Logic now.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080404210724693