Hi
I use a program called Any Video Converter to do my conversions from one video format to another.
It's a free program, but I haven't done WMV to WMA so I'm not sure that it will convert a video file to an audio file, but you can take a look at it and see if it will do what you want.
Any Video Converter - CNET Download.com
I'm not familiar with this software but it says it will do it.
Convert WMV to WMA - WMV to WMA Converter Free Download
The few times that I needed to strip the sound track from a video I opened them in Adobe Premiere Elements and then just output the Audio File alone.
Mike