No surround sound from computer to home theatre via HDMI

Hey so I have a laptop which I connect through HDMI to my home theatre system consisting of a projector and 7.1 audio. The thing is, I've configured the system in the sound settings of my laptop (windows 7) so that the 7.1 surround sound DOES work in the theatre system- I tested each individual speaker through these settings so I know it's not a problem with connecting the pc to the theatre. I've also checked and the system supports DTS.

But, when I try to play some HD videos with surround sound (like How I met your Mother from iTunes), I only get sound through the front speakers. I use VLC player and I've tried configuring the audio settings in VLC but I can't seem to get any audio from the back speakers.

I'm kind of new to this stuff so any thoughts would be appreciated. Thanks.