5.1 sound not working on iTune Videos m4v (quicktime)

Hi, can anyone help with this? Im a novice and completely stumped.
My homebuilt PC (Win XP home, 2gb RAM, AMD 2.6GHz dual core) has a Creative Soundblaster Audigy 2 soundcard attached to a Logitech 5.1 surround system. Sound quality using iTunes is awesome. However when playing m4v files (eg songs with videos) it reverts to stereo sound through front left and front right speakers, plus sub (eg 2.1)
I understand that iTunes videos uses QuickTime, so i tried playing an iTunes song (that normally plays fine in 5.1) through Quicktime - it plays in 2.1 .
This seems to indicate that Quicktime is not set up for surround sound - on the Quicktime control panel (Audio tab) it lists sound output setting (Rate (KHz) / Size (bits) / Channels (speaker set up), setting are 192KHz , 24 bit, 5.1 surround. Yet it still plays stereo.
So I guess my questions are:
are m4v files from iTunes in 2.1 or 5.1? (noob quessy I know)
can Quicktime (version 7.4.5) play 5.1 surround and if so, whats wrong with my set up?

Any ideas? Thanks for your help. I may have gone mad but I am sure the videos used to play in full 5.1 before the last iTunes release. I tried asking on the Apple support forums but got no replies.

Cheers! :confused