There are (at least) two different problems being referred to in this thread.
Problem #1) The Realtek HD drivers for Windows 7 -- at least the 64-bit version -- for some reason default to a 5.1 speaker setup having "side" speakers instead of "rear" speakers, but the driver defaults to the third speaker port being mapped to the "rear" speakers. This means that if you set it to 5.1 speakers and plug 5.1 analog outputs into the default ports on the MB, the rear speakers will not be getting any sound.
To fix that, just go into the "Realtek HD Audio Manager" (under Control Panel) and change the jack where the rear surrounds are plugged in from "Rear Speaker Out" to "Side Speaker Out". Changing the speaker setting to 7.1 will also enable them as rear speakers, but this may sound weird if applications like games or DVD/Blu-Ray movies attempt to play true 7.1 audio, since then the side speakers will be missing.
Problem #2) The Realtek HD drivers for Windows 7 do not currently appear to offer a stereo upmix feature, which is what allows 2-channel stereo to be played back on more than two speakers. So even if the rear speakers are recognized in the test utility, when you play normal stereo audio sources, sound will only come out of the "front left" and "front right" speakers (unless you use a program that has its own upmix functionality). This seems likely to be a bug: some of their earlier drivers offered this, and the drivers offer many *other* modes that upmix and modify stereo audio output, just not a simple surround upmix.
To fix this:
Ideally, Realtek should add back in the "surround fill" option that was there in some of their other (non-Win7) drivers. If that option is there, just check it and you should be set. You might want to try the latest drivers from the Realtek website to see if they have fixed this. As of 7/21/2010, the latest drivers (R2.50) do not have this option, at least in Windows 7 64-bit.
If that option is not there:
They *do* have another mode that will do this, but it may slightly affect the sound output. If you go under the "Sound Effects" tab in the audio manager, changing the "Environment" setting to "Room" will effectively cause a stereo surround upmix. (All of the modes will do a stereo upmix, but most of the other ones apply various effects that will make your music sound terrible. This one is close to neutral.)
Another option -- if your motherboard's chipset supports it -- is to use Dolby upmixing. This *also* may slightly affect the sound quality, although probably less than using their effects panel. To do this, go to Control Panel->Sound, select the "Speakers" output device, and hit the "Properties" button. This should open up a tabbed window with a bunch of settings. If you have a "Dolby" tab, go to it and check the "Dolby Prologic IIx" checkbox (or whatever option is there). This will remix stereo sound into 5.1 or 7.1. If there is no "Dolby" tab, then your motherboard does not support Dolby upmixing.
Another possible solution is to set all the jacks where your speakers are plugged in to "Front Speaker Output". This will cause the driver to send the same 2-channel output to all the speakers. However, this will break anything that tries to play real 5.1 or 7.1 audio, since the side and rear audio is then not going anywhere!
Realtek also offers a utility on their website called "3D Soundback". This may be helpful, although it has to be configured on a per-application basis. It is also designed for Windows Vista, so it may not work properly under Win7.