5.1 Audio Splitting?

Hey, I had a question for the computer savvy of our fine forums. Ok, so this is going to come across as confusing for a moment, but try and bare with me:

From my Xbox, I have a fibre optic cord leading in to a mini-amplifier, which then leads to my sound system via a trio of 3.5mm jacks (Headphone jacks).

I'm also looking in to running my computer 5.1 now, using the same system (As two redundant sound systems seems a touch ridiculous). As it stands, my computer hooks up to my TV by HDMI cord, where the TV outputs a stereo signal. From my TV, the only 5.1 output is HDMI, which neither the mini-amp nor the stereo system can accept (And I have no desire to replace either).

The sound card I was looking in to (As my video card doesn't leave many options in the way of audio output) has a trio of 3.5mm jacks too, or another fibre optic output. The only problems? I have only one fibre optic port in the mini-amp, currently taken by the Xbox. And the speakers only have one trio of 3.5mm's, currently leading to the mini-amp. Basically, does anyone know if it's possible to 'split' the signal either leading to the mini-amp, or the speakers? I don't want to go nuts and buy a new head-unit for the system, or god forbid, an entirely new system, so any help would be appreciated.

For the more "visual" people, I have three incredibly crude drawings (Sorry, it's MS Paint and I'm using a laptop, in my defense) to demonstrate my point. Blue means fibre optic, red means 3.5mm jack. Arrow means connects in to.

The way it is now:

Unfortunately, I can't seem to post the two "option" pictures (The uploader keeps failing), but essentially, the option is either to split at the blue cord, prior to the mini-amp (Meaning the already-loud PC signal would be getting further amplified by the mini-amp), or to try and split at the red cords, prior to the speakers (Meaning I'd be needing to split three seperate cords).

Any advice?

Before I address the item, I think you may have misread... 3.5mm jacks ARE good for 5.1, each one runs two channels. You have front left and front right on one, rear left and rear right on another, and centre/subwoofer feed on the last.

The system is a true 5.1, I promise you that much.

Now, looking at the item, I'd have to convert to analogue cords along the way, which not only degrades the audio set-up but will cause me to go effectively 3.1...