Welcome to the forums!
What software are you using trying to get sound from the rear speakers?
If it is a stereo source (music, many movies, Internet, etc.) it will only play through the front speakers, which is normal.
As your speakers are working within the windows control panel and realtek sound manager, your rear speakers should be working fine with a proper 5.1 source.
Either try playing a 5.1 movie or look for a surround sound test file to check the function.
A quick googling located several Promising Links
for surround sound test files.