5.1 sound works on music but not on video and TV (1 Viewer)

[kasperbs]

MediaPortal Version: 1.2
MediaPortal Skin: B3 wide
Windows Version: Windos 7
CPU Type: 4x2500
HDD: 3x500
Memory: 4GB
Motherboard: ASUS PQ/R
Video Card: nVidia Geforce 9850GT
Video Card Driver:
Sound Card: Builtin
Sound Card AC3:
Sound Card Driver:
1. TV Card:
1. TV Card Type:
1. TV Card Driver:
2. TV Card:
2. TV Card Type:
2. TV Card Driver:
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec:
MPEG2 Audio Codec:
h.264 Video Codec:
Satelite/CableTV Provider:
HTPC Case:
Cooling:
Power Supply:
Remote:
TV:
TV - HTPC Connection:

5.1 sound works when our music plays, it works in TV-mode sometime (i guess when the program is distributed in 5.1) but it doesn't work in video, maybe because the files does not include 5.1 sound.

The problem is that when we view a video or tv program that doesn't play in 5.1. There's only sound in the front speakers. My question is, if there is a way to have all speakers play sound, even when the file is not in 5.1?

I know very little about all this, so hopefully someone can tell me if there is a setting to fix this.