For video converting use SUPER its free n powerful
also it can convert video to sound only, try it
Well the whole point of playing WMVs would be of the video+audio. All WMVs I get I wish would play 'out of the box' with ZMplayer or stock mplayer. It's a pain in the ass and time-consuming to convert them really, but for now without reflashing this is the only way.
About SUPER: I've heard many positive reviews of SUPER, but also a few bad ones. Some of the complaints include SUPER having spyware, or violating GNU-GPL licenses or whatever, which I will not support. Besides, I mess around with video files a lot and there's nothing better than knowing the exact encoding configurations.