The fact that you say that you can hear sound from all of your speakers when you are testing them through Windows Sound Manager would indicate that the problem is not hardware but software based. Also, you say that you have tried three different versions of Windows OS which all exhibit the same result. This is very unusual and if you are sure that your 5.1 speaker system is functioning correctly then this indicates even more firmly that the problem is one of software.
Without knowing how your PC is setup and the software you are using, solving this kind of issue is almost impossible.
As I said previously, if the problem is present when running a Windows based system, try running a Live install of a Linux based OS. This can help to confirm if hardware is the issue as, if everything works OK in a different OS then the problem really has to be based in the Windows environment. Also, Linux uses different media players and drivers to the Windows versions which can aid in troubleshooting and helping to narrow down the root cause of the problem.
What software are you using to play audio?
Is all of your software legitimate or are you running cracks?
What web browser(s) do you use?
Which anti-virus/malware protection are you running?
Are you sure that the on-board sound is set as the default output device in all of the software you are running?
Does the software you are running have the correct codecs for playing back 5.1 sound?
All of these things and lots of others, can have a bearing on why audio is not playing back correctly and to solve the problem, you will have to look at and consider many things, not just the motherboard, as a possible cause.
