5.1 home theater only gives 2.1 output

Solution
Try connecting the white and red RCA cables from R and L to FR and FL.
Then switch to 5.1. You should get sound to the front speakers.
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yes i have 5.1 sound on my pc i checked it my refering this "" start>right click on computer>select properties(at the bottom of list)>somewhere at the top left you will see "device manager" click it>look for "sound, video and game controllers>click the sub category that pops up under it> go to details>where it says property click on it>scroll down until you find "lower logo version>it will tell you if it is 5.1 or 7.1 or 3.1, whichever it is. ""
 
Then try using this kind of cables for connecting the PC to the 5.1 back panel
https://mycablemart.com/store/images/products/1806_small.jpg , and you can test then if getting sound through all the speakers.
What I mean is that you need 5.1 input for 5.1 sound.
If the white and red cables that are now connected are coming from a TV, what you can do is using a splitter ( something like http://www.computercableinc.com/ccinc/images/CABLE-Audio-Y-Splitter-2-RCA-Male-to-1-RCA-Female-Cwholesale.jpg ) for connecting the BR and BL ports too. The only issue would be that you'd get the same sound from the front and back speakers and not 5.1 (different sound).