optical output problem.....

I recently bought a Beresford DAC (7510) to use between my laptop (sony vaio ar51) and my preamp (naim 202). Previously I had an external sound card connected to my preamp via the audio out and a usb port.

Anyway I use the laptop to play movies not listen to music, so I have it connected by HDMI to an Infocus IN78. Everything worked fine beforehand but I had to connect in a ground loop etc to minimise background hum and so on and I was hoping a DAC would improve the sound and I'd also end up with a little less mess re: wires.

So, I dutifully connected in the DAC and using a TOSLINK optical cable with a mini adaptor at the laptop end. Using windows media player I can play the audio files fine. However, when I play a movie either from my hard drive or direct from the DVD/blu ray drive, I get no sound rather just a kind of hum/white noise. Same with Intervideo. As far as I can tell all settings are as required with SPDIF set as the audio output.

Any ideas where I'm going wrong. One thought I had was may be the HDMI is blocking sound from DVDs perhaps assuming that this goes to a proper AV amp and therefore processing audio and video?????

Any help would be much appreciated....

Thanks

Andy