Tim, it would work. All you need is 3 of these cables - stereo phono to jack - http://www.amazon.co.uk/3-5mm-Jack-Phono-Audio-Cable/dp/B000Q6LSWM/ref=sr_1_2?ie=UTF8&qid=1341091267&sr=8-2 The jack ends plug in your PC speakers. The phono ends plug in to the Yamaha's pre-outputs - One set: front L&R, the next set: rear L&R, the last set: centre and sub. It's just like hooking up your PC sound card to your speakers except one end is stereo phonos instead of stereo jacks...That's it.
You wouldn't need to run the PC all the time. You wouldn't need an optical switch. You wouldn't need a device to convert coax to optical. You wouldn't have two sets of volume controls (PC software + speaker volume) and you wouldn't lose sound when the PC goes to sleep.
If you're not prepared to spend a little money to get something that will do the job 100% then that's a different thing, but the kit and method of connection will work for you 100% I can guarantee it.
BTW, your Amazon web links don't seem to work.
Original thread and reply: https://answersrip.com/question/index?qid=20120630074139AAeG0dU
[Tim wrote] "Thank you for the answer below - that device would not work for me. The 5.1 speakers I have connect using only 3 3.5mm jacks. Front stereo pair, rear stereo pair and centre + sub. That needed 6 separate cables for 5.1"
[My original reply] "Sorry to be the bearer of bad news but I am afraid it is more complicated than you first thought.
The PC speaker inputs are split across the 3 jacks - Front L & R, Rear L & R, Centre & Sub. Even if you had a box to convert digital to analogue (the box is called a DAC) then connection to just one 3.5mm jack means that only one set of speakers works. e.g. The front left & right would work, but the rears, the sub and the centre wouldn't. You need to have all three jacks connected and fed with signal for the speaker to work.
PC 5.1 surround speakers rely on the computer audio card to generate the signals for the 3 jacks. Having the PC do the processing saves money. This is why PC speakers are so much cheaper than a proper 5.1 surround system.
To make your equipment work in the way you want you would need an AV Pre-amp. This would take the digital (Optical and Coax) and analogue (red & white phono) signals in, then do the necessary conversions and decoding, and then create signals to drive the three sets of 3.5mm jacks. Unfortunately the current generation of these sorts of pre-amps are very much at the top end of the AV market. Prices range from £800+ However, there is a solution if you don't mind secondhand.
Yamaha made an AV pre-amp for people would wanted to add surround sound to their existing stereo system. This would be absolutely perfect for you. Search Ebay and the classifieds for the Yamaha DSP E800.
The E800 will do everything you need. It has 3x optical inputs, 2x coax inputs, 3x analogue inputs. It decodes digital signals in Dolby Digital and DTS formats - so that covers your DVDs, Sky Movies, Sky HD channels, PS3 & XBox games consoles, Blu-ray down-converted audio and PC signals in digital format. It also does ProLogic decoding for stereo TV sources and DVDs and games in analogue Dolby Surround. You even get surround effects processing for turning plain stereo in to surround. Other creature comforts include remote control.
As a side benefit the signal decoding and processing will be far far superior to anything you can do from a lashup of mix 'n match boxes cobbled together to make your PC speakers work. I'll also bet that the Yamaha will work out cheaper too. Just add some stereo to 3.5mm jack leads for hooking up the speakers and a few digital leads for the sources and you'll be in business.
DSP E800's sell for around £25-£50 on Ebay. For someone in your position and for what they do that's a bargain. Here's one currently listed: Ebay item number 221054903555"