Desktops: help. my 5.1 sound does not work on my Dell pc

help. my 5.1 sound does not work on my Dell pc

I have a Dell dimension 9100 with windows xp media center edition. It came with 5 speakers and subwoofer with analog wiring.I have windows media center, vlc and powerdvd 7 to hear dvd.
Now the problem: My 5.1 sound has never worked correctly. I can hear the background noises: music, sound effects,BUT THE DIALOG IS EXTREMELY LOW.
I've troubleshoot with Dell on the phone and they made me reinstall powerdvd, reinstall the sound drivers, go to safe mode and go to volume control,etc AND I CAN'T FIX THE PROBLEM. When I go to Creative Diagnostics the sound works fine and I hear sound from all speakers and is all same level.In Windows media center settings I hear sound from all speakers but some is way louder than others (center channel is very low in media center)

While troubleshooting I noticed that the sound works great if I change the setting to 4 speakers but with 6 speakers I can't hear people talk! I have played with the individual spakers in powerdvd but it does not fix the problem either.
So, now Dell wants me to erase my pc and reinstall from scratch as they think windows might be corrupt but I really do not want to!!!!
Can anyone tell me what to do. Any suggestions? does anyone know what could be creating the problem?

I would appreciate any suggestions!
Thanks