It's almost gaurenteed to be that you're lacking the AC3 filters for wmp. Google for AC3 Audio and you'll find plenty of what you're looking for. this is one of the first sites that google spits out.
see if this is any help. not all divx are encoded with ac3 audio, so most would be able to play with any mp3 codecs, but the ones that require ac3 will react like yours are.
http://www.divx-ac3.com/ac3_filter/
good luck.