that would not be true surround sound. its "faked". if you're happy with it then thats great, just letting you know its not true surround.
i cant say for sure but it might work with the line in. its easy enough to test... plug a mp3 player or phone into the jack with a male to male 3.5mm cable and see what your speakers act like with it (and check in settings if they only play stereo)
if that works and you get your "faked" surround then....
you could get the line in signal easy enough from something like this
http/www.ebay.com/itm/HDMI-to-HDMI-Optical-SPDIF-RCA-L-R-Audio-Extractor-Converter-Splitter-for-TV-/221412518597?pt=US_Audio_Cables_Adapters&hash=item338d36eac5
only thing to note is that i'm not sure if the xbo needs hdcp support or not. i quickly glanced and it seemed like it didnt for gaming but would for movies. im not sure if the extractor i linked has hdcp support (but if the xbo doesnt need it for gaming then it should be fine)
however... keep in mind that this is only faked surround from a 2.0/2.1 source. true surround is more beneficial for positioning but if all you wanted is "sound output" then it would do it. (if you want true surround on your pc speakers... this isnt the way to go about it)
there is another way to go about it.... instead of trying hard to get the pc to recognize the input... you could completely bypass the pc but still use the computer speakers. if you use a 5.1 audio extractor like this
http/www.ambery.com/2hddodtsdihd.html which would allow you to connect via hdmi, with hdmi passthrough to the tv for your console... you can then plug your 3.5mm plugs from your computer speakers into this set and get sound (since the speakers are self powered). its $128 however you're going to spend that (or more) if you try to get a game recording card which is more clunky to setup and might have speaker delay. this is the better choice. if you wanted to have both your pc and xbo connected at once to the speakers you can always get some stereo male 3.5mm to 2x stereo male 3.5mm Y splitters. one for each wire your speakers have (3x splitters for a 5.1 or 4x for a 7.1)