Oh funny, I was typing my last post just as you wee answering me from my other post. I guess then we're not sure what it is. I was hoping it was the fixed audio but clearly that's not the case. Like you said there's no .nfo so there's no telling what it could be.
EDIT: I just downloaded the first part to see what it is and upon opening it I can see it's a VIDEO_TS folder so I guess this will be a DVD. Oh well. I've got the mkv I'm just waiting for a fixed 5.1 track.