well, seeing the slug is ARM, there isnt really a integer-based implementation of a wma-decoder floating around.
I know some folks over at roku have been tinkering with one, but its not realtime, afaik.
Some folks over at rockbox.org have been trying to get it to run real-time, but I havent heard of any progress for quite a while.
So I think the slug really lacks CPU horsepower to decode WMA, for now.