there is dvd ripping software available to let you turn the audio track on dvds into wav files or mp3s. i can't remember the one i used last (it was a 30 day full working trial) but you could select which stream you wanted to rip, (ie. DTS, DD, directors commentry, spanish, french, etc) and you could also choose what channels you wanted and if you wanted them seperated or combined into a stereo track. thus, you can rip the vox and have an acapella track, which is what you want to do, or you can rip the left and right fronts and leave the rest, giving you the backing music to some movies (which is what i wanted to do)
i think it was called something obviously like easy dvd audio extractor. you should be able to google it up. you might find it on
www.videohelp.com - a great site for techy stuff and links to download sites
with the dvda or sacd, the easiest thing is to output direct from dvd/cd player and just connect the phono leads to a cd/dvd recorder or minidisc, recording the channels you want (my trick for isolating stuff in years past), altho i think there is dvda ripping software available now
i'm just listening to bowie's serious moonlight tour now actually!