I don't guessed it, I deduced it - my work need such skills (IT) ;)

Finally I found time to check Your solution and it really WORKS!

I tried this on Windows 8.1.

Everything what is need to do:

1. Copy xaudio2_*.dll files (in my case 9 + 9 = 18 files) from Windows/system32 and Windows/SYSWOW64 folders and paste it e.g. on desktop.
2. Hex edit those copied files with e.g. freeware tool "HxD Hex Editor". 2.1. Open each file, 2.2. click "replace" (CTRL+R), 2.3. in FIND box insert "4D069FF12C08274EBC736882A1BB8E4C00000000" (without quotes) 2.4. in REPLACE box insert "260E87E4C53CD24CBA46CA0A9A70ED0400000000" 2.5. click REPLACE ALL 2.6. save each modified file with "save as.." option to new folder called e.g. "Modified" (so source file could be a backup).
3. Now if you try copy modified files directly to windows (source) folders you will get error becouse you don't have privileges to do that. First you have to "take ownership" of those system files in windows folder.

Use this tutorial:
http://www.tech-recipes.com/rx/7168/how-to-overwrite-system-files-in-windows-7/
Be very carefull - you're gonna work on system files. I don't take any responsibility for your actions.

4. When you run (in my case) 18 scripts in CMD and took ownership of files, now you can copy modified files to the windows/system32 and windows/SYSWOW64 folders and replace original files.

That's all - I restarted PC and ARMA 3 works like a charm with 5.1 surround sound!

Really thanks for this solution, now I can enjoy this WONDERFUL game in 100 %!

I think You can close this issue, is solved, thanks!

Yes, my sound card supports 5.1 DD surround sound, look at attached photos/screens:

1. Realtek HD Audio.png - You can see Dolby Digital Live option enabled,

2. Windows sound settings.png - same as above,

3. SPDiF connection to laptop.jpg - You can see optical cable is connected to laptop.

Here is specification of laptop:
http://shop.lenovo.com/us/en/laptops/ideapad/y-series/y580/#techspecs
You can find in "I/O Ports" section that this laptop has SPDiF port.

4. Denon amplifier.jpg - You can see, that amplifier has DTS and DD decoder. Photo has been taken when ARMA 3 was running, You can see that amplifier recognize 5.1 DD sound becouse DD signal light is red, display shows too that Dolby Digital sound is send to amplifier.

As I mentioned before, Witcher 2, Skyrim and Battlafield 3 works on this settings perfectly.

I bought ARMA 3 when it was in alpha stage and I don't reinstall it from this time. Would this might help?

I'am BIG fan of Your games from Operation Flashpoint and it would be awesome if I can figure out from what direction sounds come from.

EDIT:
@RobertHammer - this issue You send link to is about false directions of sound, but this sound CAN BE HEARD in all speakers. My problem is that rear speakers don't work at all.

UPDATE:
I uninstalled ARMA and deleted all related files, then downloaded game again and unfortunately no change.

I read somewhere that ARMA 3 is using windows sound settings in it's sound engine. Look at this screens:

1. Windows sound configuration.png - You can see, that there is possibility to configure laptop build-in speakers.
2. Windows sound configuration 2.png - If changed to Realtek digital output (which is used to send 5.1 sound to amplifier using optical cable) there is no possibility to change audio channels, this option is grayed out.

I'm wondering that this is a cause of problem. Perhaps ARMA 3 is looking for sound configuration in windows settings, but find out only information about realtek digital output which is only passing-through sound and can't find information that there is 5.1 channels, so game sets stereo channels by default.

There is any solution for this problem?