Use all 5.1 speakers with a 2.1 audio source

This might not be exactly what you're after, but there is a Winamp plugin for Windows Vista/7 that provides various options for upmixing stereo signals to 5.1 (and higher), you might just be able to make out the configuration options from this blurry screenshot:

In my case, I was having the same problem, my music and most videos were on 2.1 and I only heard 2 of the 5 speakers. Videogames and some videos had 5.1 so no problem.

If you're running Win7 and you have no other than the default sound driver, there is an option "Speaker Fill" that solves all problems!

First, right click on the sound icon in the task bar (right lower corner), choose "Playback Devices".

If you have only one sound output, it'll be the only one on the list. If you have more than one then, while playing some music, you'll notice one of the device's volume bar moving, that's the one you should select.

Select your audio input and click on "Properties". Then go to "Enhacements" tab. There should be a checkbox with the "Speaker Fill" in the middle. Select it and press apply.

Worked for me perfectly, 5.1 sound on 2.1 music =D

Usually with a 5.1 speaker system on the Realtek HD,
you should get an upmix of your stereo and 2.1 audio.
Check this AVSForum discussion on Disable stereo upmix on Realtek HD Audio?

The Reaktek control app has options to select the Audio I/O for various number of channels. Maybe you need to change this to 5.1?

If you are using Windows Vista, i came across this weird discussion on
Vista and Realtek HD audio cannot provide surround sound