The Mac G4, or to be more specific, the Mac OS9.1, does not _yet_ support Dolby Digital Surround. Only Dolby Pro-Logic Surround, which is an analog format. (Funny, the DVD Player 2.7, (Released today), splash logo has the Dolby Digital Logo in it- so Apple is paying a licensing fee for a feature not yet supported. Go Figure...)
The other hardware options mentioned here don't work with a Mac. They could, if Apple or somebody wrote the MPEG to USB/Firewire bridgeware, but that hasn't happened yet.
A solution maybe coming; from the land of the Rising Sun, Sony-side up.
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