Logitech Z5500 and s/pdif coaxial digital - problems

[Aznboy]

i got logitech z5500 speakers and i connected it to my pc through the onboard coaxial s/pdif out. The sound is good, but i have some problems.
1. I only get surround sound when im playing a dvd with DTS. I dont get surround sound on Doom 3, which is 5.1. I really want to have surround sound on Doom 3, and its only coming out stereo. THis is all through the s/pdif.
2. Also, the speakers do not sound always when im opening forlders and stuff(sound that is for a split second) because it says " no digital data" when the speakers are inactive, and by the time it takes to become active again, the sound is gone. Why are they so unresponsive that way.

I only have the onboard soundcard from my ASUS P4C800 Deluxe MOBO; so, i only have 2 channel sound card. thus i do not wanna go through minijack to get surround sound.

PLz help me get this to work. If it will not work this way, wut should i buy?
Do i need a sound card like a Creative Audigy 2 ZS? Or am i dont something wrong?

[Aznboy]

plz respond quickly, i need help quickly