Well, I've been ripping all my DVDs and Blu-rays to my harddrive (you'd be surprised how many times the audio from the DVD/Blu-ray is different from the CD version, whether it's edits from the songs, chopped out banter between songs, or simply different versions/nights). 98% of the shows I've done have the stereo mix provided. It's my understanding (I got this from a Steven Wilson interview about... one of the Yes re-releases I think, though maybe it was Marillion) that you need a stereo mix to start from for a 5.1 mix. I think you might find a stereo mix already there for you; if so, Cocopjojo's process just got shorter: rip the song from the blu-ray with MakeMKV, then convert it to your file type of choice, and viola!