Unless the track is 5.1 it will only come out in 2.1
You have two ways of getting 5.1 sound:
1) if you have PowerDVD, put your files in that and then use Dolby Pro Logic IIx or DTS ES to get it to fill the extra speakers. Only problem with that is that it will only work when using that program - and other sounds (i.e. Windows sounds, Youtube videos, etc) will still be in stereo
2) As most of the new Realtek HD Audio cards have, use the Speaker fill function. You will need to have the newest Realtek HD Audio drivers though and use their control panel to fine tune the sound.
For this step, ensure the speakers are on 5.1 - there should be an option for "Speaker Fill"
Whilst you are here I would also recommend turning on Bass Management so that all the low frequencies are managed correctly and redirected to the subwoofer - similar to how many home cinema receivers do it.
Using Room correction you can adjust how powerful the bass is (I put the speakers on +0 DB - +2DB and then have the Subwoofer are 6DB minimum to get that nice impact without it being too boomy). as well as adjust the distance between the other speakers for optimum effect.
Should you need to the get the latest drivers from Realtek here's a heads up. They don't have any Windows 7 explicit drivers for some of the chipsets (mine for example, the ALC886). In which case, download the Windows Vista version (32 or 64) and then use the Program compatibility wizard to kid the program into thinking it is running under Windows Vista with Sp2. Works seamlessly
Good luck!