Vista - Audio issue

Hi everyone,
I've built a HTPC for myself, but I'm having issues with sound in Media Center.

I recently bought myself a Philips SRM 5100 remote control for work with WMC, and they work great together (pior to that, I had never used WMC - which now, as I understand, seems like a great piece of software).

The problem is: when playing back a DVD (or apparently anything mixed in surround), I don't get all the channels to play along. When I switch to 2.1 sound, I don't get the voice tracks. When I go for 5.1, I don't get background/ambient details and the subwoofer/low sounds are really weak.

I tried a different software (XBMC) for a test, and sound works ok with it (but the remote doesn't), so I guess there's something wrong with WMCenter, its configuration, or codecs.

Can anyone help me solve this?

Thank you!