Swap audio track on a WMV without recompressing it?

Could someone give me (somewhat detailed) directions on how to switch out a WMV file's audio track for another without recompressing the file?

I can make the new audio track exactly the same length beforehand. I tried using ASFbin but that just said "general compatbility error" when I tried to add the new WMA file I'd made for the purpose.

I've heard Expression Encoder bandied around - what exactly would I do within that program to achieve this result?

Thanks