the only thing i would think of is to open it up through a video editing program and extract it. i only use pinnacle though so i dont know of any free ones
I'd give virtualdub a shot. Maybe you can just save the audio after you open the file. I know it is easy with a regular AVI file although I'm not sure if it does WMV files.
I found a program just using Google searching for "Extract audio from avi files", found a few programs to do the trick but since they weren't free it would only do 20 seconds worth which was fine for me. But if you want minutes of audio well...either buy it or crack it.