Correct, the PowerMac has a stereo analog out, and a multichannel optical out.
In order to get multi-channel sound you either need speakers or a receiver that has an optical input and a Dolby or DTS decoder,
OR
you need an outboard USB decoder such and a M-Audio Sonica Theater which can feed 5.1 = 6 channels (23 stereo jacks) of analog audio to a speaker set that does not have a built in decoder.