SB Live 5.1 PCI card and control panel configuration

Hi,

I have an SB Live 5.1 PCI soundcard (0100) with a Logitech z-640 5.1
surround sound speaker system; not the greatest or the most expensive
system on the planet, yet fully adecuate for my needs.

Using WinXp in the ControlPanel/sounds&audio area, I have it
configured for "surround sound speakers" and everything works fine. If
I configure it for "5.1 surround sound speakers" then weird things
happen. Like when I watch a movie, I'll only get the background
sound-track and no dialogue track.

Can anyone orient me here?

thanks, eric