I have the MSI P45 Platinum motherboard WITH A DIGITAL OUT SPDIF (optical fiber).
Connected to a Sony HT-DDW685 with the same spdif
BUT THE SOUND IS JUST STEREO!!!!!
why?? if the motherboard has 8 audio channel and the HT can receive blue ray for this entry.
Spdif is usually used for either 2.0/2.1 (digital PCM), or 5.1 (Dolby Digital/DTS), or through various upmixing tecnnologies (Pro Logic/Dolby Digital Live/DTS Interactive).
Basically, unless the output program in question supports upmixing, or unless you have a reciever that does the upmixing for you, you only get 2.0 as the output.
Also note, Windows has some issues getting beyond 2.0 using Spdif, as Windows expects a 2.0 as the output. Make sure that (I'm assuming Vista here) 'Speakers' is the selected output with 5.1 selected as the speaker setup, not 'Digital Output', which assumes 2.0. (Stupid, I know).