Why not filter out wma by putting .wma into your keyword filter. Results can vary from day to day or week to week. Check some of the tips here http://www.gnutellaforums.com/showthread.php?t=63483

autogenerated spam results & autogenerated mp3 spam results, you can use your junk filter for these. After doing a few times the filter learns to filter them out. Just be sure you filter out bad stuff & not good or your filter will be learning the wrong things.