DIY surround-sound using multiple sets of speakers - need help

[OP moeburn 53]

So for a long time, I had my old Sound Blaster Audigy (yes the original Audigy from 15+ years ago), a nice pair of massive tower speakers for the front L/R, and a pair of decent small computer speakers for the rear L/R.  The SB's drivers were perfectly capable of this setup - all I had to do was go into the Windows Playback Devices dialog, and tell it to configure the SB output as "quadraphonic":

 

 

It worked great for games and movies no problem - I could hear enemies sneaking up behind me on my rear speakers in multiplayer shooters, and I could hear the rear effects in movies as well.

 

But the SB died, and I'm left using my onboard "Realtek High Definition Audio".  Like the Audigy, it has 6 plugs, 4 of which are output, and I think only 3 of which are analog output - so it should be perfectly capable of DIY quadraphonic:

 

 

But for some reason, it refuses to work using the "Quadraphonic" option in the dialog - nothing comes out of the rear speakers at all.  The only way I managed to get it to work on the onboard sound was by selecting "7.1 surround" on that dialog, and then on the next page it lets me uncheck every "optional speaker" except for the rear left/right.  While this does work, and sound correctly comes out of my rear speakers, there's a problem - Games and movies now think I have 7.1 surround speakers, and not 4 speakers, and so I miss out on pretty much all the dialogue in games and movies because it gets mapped to a center speaker that I do not have.  I can fix that in movies by using the custom speaker mapping option in my movie player, but not in games - there is no way to remap the center speaker to the front L/R in games.

 

So can anyone tell me either A) if there is a way to get the "quadraphonic" option working using my onboard card, or B) if there is a way to DIY a center speaker?  I have no idea how that would work, or if it would even be possible - Is one of those 6 plugs for a center speaker?  The only sets of powered speakers I have are stereo; if there is a plug for the center channel, could I just plug a stereo speaker into it and hope for the best?

 

Thanks for any help, folks!