I have an Asus A8n-SLi Deluxe motherboard with a nForce4 chipset. I have an optical cable running to my Samsung upscaling DVD player/surround sound (HT-Z310). But the sound only comes out of two speakers. Can someone help?

On the Samsung remote, is there a button called surround or auto? Try hitting that. It may be in stereo mode.

Also, check your computer settings. This is probably the culprit. You most likely have to set your computer to send surround instead of stereo.

The audio hardware can only encode 2 channel stereo through the optical spdif. But it should have multi channel pass through.

So if your playing dvds or video files that already have a 5.1 source you have to look through your software options and stuff to just pass through the audio without the hardware trying to decode and encode it. For example if your playing .mkv video files I use ffdshow audio decoder. Go into the setting and under codecs you'll see AC3 and DTS set them both to spdif. Now you'll have Dolby Digital or DTS going to your reciever. Note: You won't be able to adjust volume or use equalizers and such through your computer.

And for pc games you won't be able to send a 5.1 signal out the optical spdif you'll have to use analog connections for that. The hardware just can't do it over the spdif connections.

Most sound cards are still like this unless you get the really expensive ones.

I do plan on playing mkv files, I was going to use VLC, what do you suggest and how would I set it up for optimal a/v quality

Have you gotten the 5.1 to work over the optical spdif connection?

Go here and download this codec pack.
cccp-project.net
You'll have to copy and paste link.

It's pretty good for everything you need.

If you are playing H.264 mkv files then get CoreAVC it works really well.

VLC is okay it works with pretty much everything but I don't really use it as I don't find it good for high quality videos and audio.

I haven't had a chance to try it again, I'm assuming it'll work okay, because the thing I tried and got two speakers was youtube. I know I should have tested more but I was really frustrated last night, my video isn't going to my TV right and I bought an optical splitter for my receiver that just flat out didn't work, and it took me a while to isolate that as the first sound problem