I don't know if some video software will capture audio only, from a video source, but if you capture both as usual, as an avi file, sound manipulating software such as Soundforge will open an avi file, and then you can just save the audio track. You an also burn to CD direct from Soundforge.
Soundforge is a useful tool anyway, as you can cut, copy, loop etc bits of music etc you may want as soundtrack, and also timestretch. There is a 'light' version see here http://mediasoftware.sonypictures.co...ct.asp?PID=945
Win XP, Athlon 1.8, G force 3 T, 1 60gig drive + 1 250 gig, Pinnacle Studio 9 plus mostly, Premiere 6, and Vegas 5 sometimes, Soundforge, Cool Edit.