I have been having quite the problem hooking up my Yamaha AMP receiver to my PC. It is a 5.1 system and i know it works because when i use the FM setting all speakers and sub play. But when i try to use my PC with it i only get sound out of the front left speaker and the front right. My HD audio deck says it is using the 5.1 but when i do the test i plays front left then front right then cycles threw the rest with no sound but shows that it is using sound. I do the tone check on the Yamaha with this setting and all speakers including sub work as expected. I have tried using HDMI into the receiver as well as aux in and the basic left right with a Stereo Male To Dual RCA Female Adapter that goes into the aux in/out put green port on the computer all with the same problem
sound playing over all speakers from the radio is not surround sound. it is stereo sound that you are just pushing to the other speakers.
windows sounds, youtube/internet and some older games are also only in stereo. you will often find surround sound only in movies, games and in music which is recorded in surround (which isnt terribly common).
hdmi, optical or component audio cables should be able to transmit the signal from pc to receiver but keep in mind that your receiver settings and windows speaker settings can influence this.
normally i keep my receiver set to auto detect. it plays stereo sound out the fronts and game/movie surround sound out all speakers. to play stereo sound out all speakers you can change your receiver speaker mode to repeat the sound on your rear speakers as well. this is not surround sound but will fill a larger room with sound.
edit: connecting only to the one 3.5mm port on a pc is stereo only. for surround sound you need to connect up to 3. the LF/RF, C and LR/RR channels. you would then need to adapt this to your receiver input. hdmi or spdif is a better choice though.
First, what are you hoping to accomplish? For instance, on my system, I only use my computer to listen to music on my audio system. I use the optical digital out of the computer, hooked up to a spare optical input of an Onkyo preamp processor. However, to get a signal out of computer requires changing the default audio signal to SPDIF out first. OR, no sound! Now, if I wanted to throughput 5.1 Dolby Digital I would use the HDMI on my video card and configure audio to pass thru as well. Very much setting configured. So, it all goes back to what you are trying to accomplish!