I'm out of my element here, but I did try extracting the audio stream from a DVD using the old DVD Decrypter. Set it to IFO mode, then choose the "Stream Processing" tab, check the "Enable Stream Processing" box, check the stream you wish to extract, and finally set the copy mode to "Demux":
After you have ripped the file, rename the .VOB file to .AC3, .AVI or .WAV or whatever. The players usually seem smart enough to figure out what it really is regardless of the extension. Which leads to the question, what is it, really? I'm pretty sure it is Dolby Digital AC3, but I could be wrong.
I also stumbled across a site with a few 5.1 AC3 sample files:
There are too many formats for me to keep track of all of them, although I would like to.