The Problem:-

Windows  Media Center 2005 with Roll Up 2 is issuing the wrong setting for 5.1 analog. Surround Sound is forced to Stereo 2-channel mode.

Resolution:-

There  is a  registry fix issued by nvidia.

Here are the simple steps:-

1]Close the Media Center.

2] Download this attached registry file MCE2005_ForceDD5_1.reg

2]Double click on it to install.

[Do not run  “Setup Speakers” wizard again in Media Center settings, that will again change the value,in that case you have to run the registry fix again]

If you want to do it manually:-

Start>Run>Regedit>OK

Now navigate to the following location and change accordingly:-

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Media Center\Service\Video]
“AudioOutputFormat”=”{696E1D31-548F-4036-825F-7026C60011BD}”