You can add words like wma and other file types to the keyword filter. In LW go to Tools>Options>Filters>Keywords, and add the file extensions you don't want in your search results. You can also add mp3 to the end of your searches, e.g. "Song Title .mp3".
Check out this thread.
How to find music. About halfway through the first post it has info on using the keyword filter. It also a lot of good tips for finding music files.