The Input is fine. I have connected the speakers to my iMac using the optical digital cable and they work just fine. It's just the Audigy I'm having trouble with since neither of the two digital connections produce any sound. It's very likely something I'm not doing right but I can't figure it out. I've re-checked everything 3-4 times and still nothing
I'm not sure if the iMac has something like this, but Microsoft Windows has a mode in the sound options of the control panel called "enable digital output" which toggles on/off digital or analog output.
Have you downloaded the latest drivers? I was having some trouble with my X-Fi and when checking Creative's site, found a driver download labeled "critical". After downloading that, my card worked a lot better.