Problem with sound using - ASRock FM2A75-PRO4-M - Motherboard

[Sam654]

PC is being used as a HTPC and consists of :-
ASRock FM2A75-PRO4-M Motherboard
LG CH12LS28 12X BD-R Blu-ray DVD Combo Drive
OCZ Vertex 4 128GB SSD
G.Skill F3-12800CL9D-4GBNQ (2x2GB) DDR3
AMD A8 5600K 4-Core Processor

The motherboard supports 7.1 surround sound.
My speakers are connected to the motherboard using optical cable and are 5.1 surround sound.
I'm using XBMC and the sound is not right. Speech in movies is very quiet and effects are very loud. I've been through the XBMC forums and they are all telling me to adjust my motherboard sound settings to 5.1, but I can not find where this is done.

I would appreciate some guidance please.

Regards
Sam

[Sam654]

Re: Problem with sound using - ASRock FM2A75-PRO4-M - Motherboard

Sorry, forgot to mention, it's running Windows8 6 bit OS.