1/8" jack is mono/stereo, the same as RCA (red & white hook ups of composite or component). the only way to feed a true surround sound single is by coaxial or optical (toslink).
Lets start with your DVD player outputs. What types do you have? If you don't have coax, optical or RCA multi channel out, the best you can do is stereo.
Now look at your input options. Do you have a stand alone surround sound receiver? If so, see it has coax or optical inputs. if it does, problem solved - use the receiver!
If you don't have a stand alone receiver and must use your PC you might need to upgrade your sound card to something with optical in.
Here's a card that will do everything you need:
http://www.xoxide.com/auzentech-x-meridian.html(just a note, that card is ~$200, you can get a stand alone receiver for less then that)
Now i assume you're only using one speaker output of your sound card to drive your 5.1 system. this will not get you true surround sound even with an optical in singal (remember the whole 1/8" jack is stereo talk.. it applies here too). Your speaker system should provide you with two or more cables to connect the output of your sound card to the systems inputs.
You need to make sure they all are connected and your sound card is configured to output the right singles on each line (your speakers should have came with software to do this or its in the driver dialog)
hope I provided you with enough information to point you in the right direction. let me know if you understand or if you're confused.