5.1 system in-built soundcard Speaker-fill not working

Good day kind sirs. So the problem is as it's said in the title. Just bought new and shiny Edifier C6 5.1 sound system. I used RCA to 3,5 leads to connect to my PC via amplifier. I have an in-built soundcard which had 3 multimedia ports: line-in, -out and mic-in. Everything's connected right and works fine during the default Realtek HD audio manager sound test (clicking on each speaker). The games and DVD-quality movies give the promised surround while the stereo-coded sound sources, such as youtube videos, music don't. I've been trying to use the Speaker-fill function but it won't help. For some reason it just doesn't copy the main front speakers to the rest.
I've spent quite a few days searching forums and the web but couldn't come up with a reasonable solution. In this forum there is a suggestion to use the register files to re-route the rest speakers to the front ones. I could use 2 register files to hop between the re-routed reg.file and the normal setup. Yet I don't feel like it should work this way. Would apreciate any help.

And I thought so as well but then I've searched the web and people suggested that it is quite an option to use "3-ported" sound card for a 5.1 systems. The problem isn't about the card not supporting it. As I've also mentioned in my post, it does support the surround sound in games and DVD-movies while not in stereo. The speaker-fill function won't work.

Exactly. They can be configured as outputs. Which I did. Still the problem with speaker-fill function persists.

So is there a solution to this? For example would buying a better soundcard solve the given problem? I mean what is the point of the 5.1 systems if the 98% of the music is recorded in stereo format? This big part of entertainment industry can't be just left out.

So about the windows function.. Are you talking about sound devices> speaker's settings> enchanments> speaker fill? Because I've been reading different posts regarding this problem and most of them say that this is the only way to make stereo work as surround. By this they mean, that this "speaker fill" copies the stereo sound from the frontal speakers to whole 5.1 set itself, creating a pseudo-surround. Because this is the function that won't work. Nothing happens when I check/uncheck it.