Creative Labs makes the SoundBlaster Extigy.
http://www.soundblaster.com/products/extigy/
It's not "exactly" Mac compatible (meaning there are no Mac drivers as far as I know) but it does work. It connects via USB to a Windows machine (not that you'd need that), but it also has optical in and out, and coax in. It has the analog outputs for a 5.1 speaker system. I'm using one because I've got multiple computers that support 5.1 audio. My only other option (yours, too) would be to buy a surround sound receiver, and that wouldn't work as well with my speakers. Hope this helps.
Regards,
Marc